This is a post about the basics of if clauses.
In clauses after if, we usually talk about uncertain events and situations: things which may or may not happen, which may or may not be true, etc.
If you’re free later, we can go for a walk.
If they’re hungry, I’ll make some sandwiches.
First Conditional (it’s a real conditional)
We use the first conditional to talk about a realistic situation in the present or future.
- If + present + will/may/must/imperative/can + verb
If we play tennis, I’ll win.
If you’re not back by 5pm, give me a ring.
Second Conditional (Unreal, imaginary situations in the present or future)
We use the second conditional to talk about unreal or imaginary situations in the present or future.
- If + past + would/could + verb
If I had more time, I’d exercise more. (But I don’t have more time so I don’t.)
*If I were rich, I’d spend all my time travelling. (But I’m not rich so I can’t.)
If she saw a snake, she’d be terrified.
If I knew her name, I would tell you.
* if I were
After if we often use were. This is common in both and formal styles
If I were taller, I’d be happier.
Third Conditional (Unreal, imaginary situations in the past)
We use the third conditional to talk about unreal or imaginary situations in the past.
- If + past perfect + would have/could have + Past Participle
If you had asked me, I would have told you.
If you hadn’t forgotten her birthday, she wouldn’t have been upset.
Zero Conditional
We use the zero conditional to talk about permanent truths, such as scientific facts, and general habits. The structure is simple:
Here are some examples:
If you heat water to 100°, it boils.
If you eat a lot, you put on weight.
If it doesn’t rain for a long time, the earth gets very dry.
If we go out with friends, we normally go to a restaurant.
If I’m tired, I go to bed early.
Sources:
Book: Practical English Usage – Michael Swan e www.wallstreetenglish.com