There are four main types of 'if' sentences, often called conditional sentences. These sentences are in two halves, with the 'if' part in one half, and the other part where you can use words such as 'can', 'will', 'may', 'might', 'could' and 'would'.
If + present form, + present form
"If you heat ice, it melts."
In this type of sentence, (the zero conditional), you could use 'when' instead of 'if'.
It's always true that when you heat ice, it melts.
If + present form, + will, can or may
"If I am late, I will call you."
"If you need me, you can call me at home."
"If it gets any hotter, we may have a thunder storm."
In these sentences, (or first conditional sentences), there is a strong possibility that the first part (coming after 'if') is going to happen. The second part says what will happen as a result
If + past form , + would, could or might
"If I got a pay rise, I would buy a new car."
"If you left your job, you could travel around the world."
"If you were nicer to him, he might lend you the money."
In these sentences, the first part with 'if' shows
that the event is unlikely to happen. We often use this type of sentence (called
a second conditional) to talk about hypotheses, or imaginary future events.
For example, "If I was/were President of the United States, I would change some
laws." But I know that I'll never be the President of the USA - I'm just saying
what I would do if I was/were in his/her position.
If + past perfect, + would/might/could...have done
"If I had revised, I would have passed my exams."
"If we had gone out earlier, we might have got to the cinema on time."
"If you had told me there was a problem, I could have helped."
In these sentences, (or third conditional sentences), the first part of the sentence with 'if' didn't happen. So there is no
possibility of the second part of the sentence happening. I didn't revise, so I didn't pass my exams, and there is nothing I
can do about it now. We use this type of sentence to show how things could have been different.