- If + present simple, .... present simple.
This conditional is used when the result will always happen. So, if water reaches 100 degrees, it always boils. It's a fact. I'm talking in general, not about one particular situation. The result of the 'if clause' is always the main cluase.
If people eat too much, they get fat.
If you touch a fire, you get burned.
People die if they don't eat.
You get water if you mix hydrogen and oxygen.
Snakes bite if they are scared.
- if + present simple, ... will + infinitive
It's used to talk about things which might happen in the future. Of course, we can't know what will happen in the future, but this describes possible things, which could easily come true.
If I study today, I'll go to the party tonight.
If I have enough money, I'll buy some new shoes.
If I finish my exam, I'll pass.
She'll miss the bus if she doesn't leave soon.
If I see her, I'll tell her.
- if + past simple, ...would + infinitive
It has two uses:
First, we can use it to talk about things in the future that are probably not going to be true. Maybe I'm imagining some dream for example.
Second, we can use it to talk about something in the present which is impossible, because it's not true. Is that clear?
If I won the lottery, I would travel around the world.
If I met the Queen of England, I would say hello.
She would travel all over the world if she were rich.
If I had his number, I would call him.
If I were you, I wouldn't go out with that man.
- if + past perfect, ...would + have + past participle
It talks about the past. It's used to describe a situation that didn't happen, and to imagine the result of this situation.
If I hadn't eaten so much, I wouldn't have felt sick.
If we had taken a taxi, we wouldn't have missed the plane.
She wouldn't have been tired if she had gone to bed earlier.
She would have become a teacher if she had gone to university.
He would have been on time for the interview if he had left the house at nine.