The short answer
Use the present in both parts for things that are always true, if plus present with will for real future possibilities, if plus past with would for imagined situations now, and if plus past perfect with would have for a past that did not happen. Would and will go in the main clause, not after if, apart from a few polite and willing uses. Unless means if not, and in case means to be prepared, not if.
How an if sentence is built
A conditional sentence has two parts. The conditional clause gives the situation, usually with if, and the main clause gives the result: if a lot of people come, we'll need extra chairs. Either part can come first. When the if clause comes first, you normally put a comma after it in writing; when it comes second, you normally do not: we'll need extra chairs if a lot of people come.
What makes English conditionals tricky is that the verb forms do not show time in the usual way. If I won a million dollars talks about the future, not the past, and the past form signals that you think it is unlikely. Cambridge Dictionary calls this distance from reality: the past form says imagined, not yesterday.
| Type | If clause | Main clause | Example |
|---|---|---|---|
| Zero: always true | present simple | present simple | If you heat ice, it melts. |
| First: real future possibility | present simple | will, can, may, might + verb | If it rains tomorrow, we'll stay home. |
| Second: imagined present or future | past simple | would, could, might + verb | If I had more time, I'd learn the piano. |
| Third: imagined past | past perfect | would have + past participle | If we'd left earlier, we would have caught the train. |
| Mixed: past cause, present result | past perfect | would + verb | If I hadn't moved here, I wouldn't know you. |
The names are a teaching convention. Real English mixes forms more freely, but these patterns cover most of what you will hear and need to write.
Zero conditional: things that are always true
The zero conditional talks about facts, rules and habits: every time this happens, that happens. Both clauses use the present simple, and if can usually be replaced by when without changing the meaning. If you freeze water, it becomes solid. When the sun goes down, it gets dark. If I can't sleep, I listen to the radio.
If you heat ice, it melts.
A scientific fact: it is true every time.
If you park here, they clamp your wheels.
A rule: it always happens.
If I drink coffee after six, I don't sleep well.
A personal habit or pattern.
First conditional: real possibilities
The first conditional is for a future situation you think is real or likely. The if clause takes the present simple, and the main clause takes will or another modal such as can, may or might: if the taxi doesn't come soon, I'll drive you myself. The main clause can also be an instruction: if you see Dora, give her this message.
The key point is that the if clause stays in the present even though it talks about the future. If it rains tomorrow, not if it will rain tomorrow. English treats the condition as a fixed point and puts the future only in the result. The same goes for when and as soon as: I'll call you when I arrive.
If the flight is late, we'll miss our connection.
A real possibility: the flight may well be late.
If Sheila rings, I might ask her to come over.
Might in the main clause makes the result less certain.
If you need help with the move, call me.
An imperative in the main clause.
If or when?
When means you are sure it will happen; if means it might. When I get home, I'll call you says you will get home. If I get the job, I'll celebrate says you might not get it. Choose the word that matches what you believe.
Second conditional: imagined situations
The second conditional imagines a present or future situation that is unlikely or impossible: if I won the lottery, I'd stop working. The if clause takes the past simple and the main clause takes would, could or might with the base verb. Nothing in the sentence is about the past. People don't complain now, so if people complained, things would change imagines a different present.
The choice between first and second shows what you think. If the bus is late, I'll take a taxi means it may really be late; if there were more buses, I would leave the car at home means you do not expect more buses.
If I were you
With the verb be, careful English uses were for every person in the second conditional: if I were rich, if she were here, if it were up to me. In everyday speech, was is also common, especially after I, he, she and it, and British Council LearnEnglish accepts both. The fixed phrase for advice, though, is almost always if I were you: if I were you, I'd take the job. The Cambridge Dictionary lists it as a B1 phrase.
If I had a garden, I'd grow my own tomatoes.
I have no garden, so this is imagined.
If you asked her nicely, she would say yes.
Imagined, but possible: a gentle suggestion.
If I were you, I'd call the landlord today.
The usual way to give advice.
What would you do if you lost your passport abroad?
A question about an unlikely situation.
The verb tells you how real the situation is.
Third conditional: a past that did not happen
The third conditional imagines a different past, where something did or did not happen, and imagines a different result. The if clause takes the past perfect and the main clause takes would have, could have or might have plus the past participle: if I had played better, I would have won. In fact you played badly and lost.
In speech both parts are shortened, and the two contractions sound alike: if I'd known, I'd have come. The first 'd is had, the second is would. A good check is the verb after it: had comes before a past participle like known, would comes before a base verb like have or come.
If the dog hadn't barked, we wouldn't have known someone was in the garden.
The dog barked, so we knew.
It would have been easier if George had brought his car.
George did not bring his car.
If we'd left ten minutes earlier, we could have caught the train.
Could have: the result was possible, not certain.
Mixed conditionals: past and present together
Real life does not keep past and present apart, so English mixes the patterns. The most common mixed conditional joins a past condition with a present result: if plus past perfect, then would plus base verb. If I hadn't met Charles, I wouldn't be here now. The meeting is in the past, being here is now.
The opposite mix joins a present situation with a past result: if plus past simple, then would have plus participle. If I were more organized, I would have finished the report yesterday. Being disorganized is a lasting fact, and the unfinished report is the past result.
If I had studied medicine, I would be a doctor now.
Past choice, present result.
She wouldn't still be working here if we hadn't given her a raise.
Past action, present situation.
If I weren't so shy, I would have spoken to her at the party.
Present character, past result.
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Would does not go in the if clause
This is the most useful rule about conditionals: will and would belong in the main clause. If you book before April 30, you will get a discount, not if you will book. If we had a bigger apartment, we could invite friends, not if we would have. If the weather hadn't been so bad, I could have gone, not if the weather wouldn't have been so bad.
Some people do say if I would have known in informal speech, but Cambridge Dictionary notes that many speakers consider it incorrect, and exams treat it as a mistake.
The exceptions: willingness and politeness
Will and would can appear after if when they mean being willing, not future time. If you would all stop shouting, I will try to explain means if you are willing to stop. If Clare will meet us at the airport, it will save a lot of time means if she agrees to. Will can also follow if when the result comes later: if it will make you happy, I'll stay home tonight.
The other exception is politeness. Shop assistants, receptionists and hosts use if with would, will, can or could to make a request softer, often without any main clause: if you would just sign here, if you would like to follow me, if you'll take a seat.
If you should see her
Should after if adds the idea of chance: if you should bump into Carol, tell her I'm looking for her. It means if by any chance. It leads straight to the formal inverted form.
Unless, as long as, provided that and in case
Unless
Unless means something close to if not or except if, and it takes the same verb forms as if: unless I phone you, you can assume the train is on time. Because unless already contains the negative, do not add another one: you'll miss the bus unless you hurry means if you don't hurry. Never put unless and if together: unless if is always wrong.
Unless is also not used for impossible past conditions: if the government hadn't raised prices, there wouldn't have been protests, not unless the government had raised prices.
As long as, provided that and on condition that
When you set a condition or a limit, English has a range of phrases. As long as and providing are more common in speech: you can play in the living room as long as you don't make a mess. So long as, provided that and on condition that are more formal and more common in writing: the bank lent the money on condition that it was repaid within six months. All of them take the same verb forms as if.
In case is not if
In case looks like a conditional word but works differently. It means doing something now to be prepared for a possible situation: take an umbrella in case it rains. You take the umbrella whether it rains or not. Compare let's take our swimsuits in case there's a pool, where you pack them anyway, with let's take our swimsuits if there's a pool, where you decide once you know. In case of plus a noun is the exception that does mean if: in case of fire, use the stairs.
I'll take some cash in case we need it on the ferry.
We do not know if we will need it, so we prepare.
We'll have to cancel the show unless we sell more tickets.
If we do not sell more tickets.
You can borrow my car as long as you're back by six.
A condition with a limit, typical in speech.
Inverted conditionals in formal English
Formal English can drop if and swap the subject with the auxiliary. This works with three words only: should, were and had. The meaning stays the same but the tone is formal, so you meet it in business letters, terms and conditions and reports. British Council LearnEnglish teaches it at C1, and notes that ordinary if sentences are always acceptable instead.
Should you replaces if you should in first conditional sentences, often in polite instructions: should you wish to cancel your order, please contact customer service. Were replaces if in the second conditional; with be you simply swap the words, were I in your position, and with other verbs you add to plus the base verb: were the economy to slow down too quickly, there would be serious problems. Had replaces if in the third conditional: had I known you were waiting outside, I would have invited you in.
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If you need any further information.
Were the sun to disappear, life on Earth would end.
If the sun disappeared: a very unlikely situation.
Had I known, I would have called you.
If I had known.
Had I not been busy, I would have come to the meeting.
Negative: not goes after the subject, never hadn't I been.
Two fixed negative forms are worth learning as wholes. Were it not for means if it were not for, without: were it not for the bridge, the town would be cut off. Had it not been for is its past version: had it not been for your warning, I would have crashed the car. A shorter alternative to both is but for: but for your warning, I would have crashed.
No contractions, no if
In an inverted conditional you never keep if, and the negative is never contracted. Had I not seen it, not hadn't I seen it, and never if had I seen it. If the sentence feels too heavy, go back to a normal if clause: it is always correct.
Conditionals at B1, B2 and C1, and in exams
British Council LearnEnglish places the zero, first and second conditionals, and the third and mixed conditionals, in its B1 and B2 grammar, and inversion with should, were and had at C1. The Cambridge Dictionary marks if I were you as a B1 phrase. In practice, B2 means moving easily between all the types, and C1 adds control of the formal inverted forms.
In the Cambridge exams, conditionals are a natural tool for the key word transformations in Part 4 of Reading and Use of English. In both B2 First and C1 Advanced there are six of them: you complete a second sentence with a given word so that it means the same as the first, and in C1 Advanced each answer is three to six words. Sentences with because, without or otherwise can often be rewritten with if, unless or but for.
IELTS has no separate grammar paper, but grammatical range and accuracy is one of the four criteria in both Speaking and Writing. A well-placed second or third conditional, for example in a Speaking answer about what you would change in your city or a past decision you would make differently, shows range, as long as the would stays in the right clause.
Why your own language trips you up
Most mistakes with conditionals come from translating the verb forms from your own language. The Romance languages build their if sentences with subjunctives and conditionals that only partly match English, so each one leads learners to a different slip.
Italian speakers
Italian often uses the future after se: se domani pioverà, resterò a casa. Word for word, that gives if it will rain. English needs the present after if: if it rains tomorrow, I'll stay home. For imagined situations, the congiuntivo matches the English past and the condizionale matches would: se avessi tempo, verrei is if I had time, I'd come.
Spanish speakers
Spanish is close to English: the Real Academia Española rejects the conditional after si, so si tendría is wrong just as if I would have is, and si tuviera tiempo, iría maps onto if I had time, I would go. The trap is the past: Spanish accepts hubiera in both halves, si lo hubiera sabido, no hubiera venido, which pushes learners toward I had not come. English needs I wouldn't have come.
French speakers
French has the clearest match. The Académie française confirms that si takes the imparfait, not the conditionnel, so si j'avais le temps, je viendrais becomes if I had time, I would come. The French trap is au cas où, which often means if, so learners write in case it rains, we'll cancel when they mean if it rains.
Portuguese speakers
Portuguese uses the future subjunctive after se and quando: se chover amanhã, fico em casa. With no such form in English, learners reach for will. English uses the plain present: if it rains tomorrow, I'll stay home; when I arrive, I'll call. Brazilian speech also says se eu soubesse, tinha vindo, where English needs if I had known, I would have come.
One trap is shared by all four. Italian in caso, Spanish en caso de que, French au cas où and Portuguese caso can all introduce a condition, so in case often gets used as a fancy if. In English, keep in case for preparing ahead.
Common mistakes
These are the slips teachers correct most often in conditional sentences, and each one breaks a rule from this article.
Wrong: If it will rain tomorrow, we'll stay home.
Right: If it rains tomorrow, we'll stay home.
The if clause takes the present, even for the future.
Wrong: If I would have more money, I would travel.
Right: If I had more money, I would travel.
Would goes in the main clause, not after if.
Wrong: If I would have known, I would have come.
Right: If I had known, I would have come.
The third conditional takes the past perfect after if.
Wrong: If I am you, I'd accept the offer.
Right: If I were you, I'd accept the offer.
Advice uses the second conditional, and the fixed phrase is if I were you.
Wrong: Unless you don't hurry, you'll miss the bus.
Right: Unless you hurry, you'll miss the bus.
Unless already means if not, so no second negative.
Wrong: We'll cancel the picnic in case it rains.
Right: We'll cancel the picnic if it rains.
You cancel only when it rains, so this is a condition: if. In case would mean canceling anyway, as a precaution.
Wrong: Hadn't I been busy, I would have come.
Right: Had I not been busy, I would have come.
In an inverted conditional, not comes after the subject.
How to make conditionals automatic
Start with the two rules that solve most problems: present after if for the future, and would only in the main clause. Then practice each type with your own life: what you do if you can't sleep, what you'll do if it rains this weekend, what you would do if you had a free year, and what would have happened if you had chosen a different school or job.
Conditionals become fluent only when you use them in conversation, where someone asks what would you do and you have to answer on the spot. In live classes with a teacher and the same small group, these questions come up all the time, and exercises with instant feedback catch a stray would in the if clause right away.
Writing is the place for the formal forms. Try a short email that begins should you need any further information, or a paragraph about a past decision with had I known. Your teacher corrects your homework, so you can try them safely before an exam.
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Questions people ask
Is "if I was you" wrong?
It is common in informal speech, and British Council LearnEnglish accepts was after if in the second conditional. In writing, in exams and in the fixed phrase for advice, if I were you is the safer choice.
Can you ever use would after if?
Yes, in two cases: when would means being willing, as in if you would all stop talking, and in polite requests, as in if you would follow me. For ordinary conditions, would belongs in the main clause only.
What is the difference between if and in case?
If introduces a condition: you act only when it happens. In case means you act now to be prepared: take cash in case you need it means take it anyway. In case of plus a noun, as in in case of fire, does mean if.
Which conditionals do I need for B2 First?
All the main types: zero, first, second, third and mixed, plus unless and as long as. The British Council teaches them at B1 and B2. Inversion with should, were and had is C1 material, useful for C1 Advanced and formal writing.
Is "Had I known" too formal for everyday English?
It is formal, and in conversation most people say if I had known. Use the inverted form in formal emails, reports and essays, where it sounds natural and precise.
Sources: Cambridge Dictionary: Conditionals: if, Cambridge Dictionary: Conditionals: other expressions (unless, should, as long as), Cambridge Dictionary: In case (of), British Council LearnEnglish: Conditionals: zero, first and second, British Council LearnEnglish: Conditionals: third and mixed, British Council LearnEnglish: Inversion and conditionals.
