IF is used to introduce a conditional clause. A conditional clause is a statement that won’t come true unless something else happens first:
“If it stops raining, we can go swimming.”
The order may also be reversed:
“ We can go swimming if it stops raining.”
There are two kinds of conditionals that “if” introduces. The first are conditionals that could happen, or might be true. These are real conditionals. Real conditionals usually use modals such as “can” or “will”:
“If you eat your dinner, you can have dessert.”
“If Sarah passes the test, she will graduate.”
The second kind of conditionals are called imaginary or hypothetical conditionals. These are conditionals that did not occur, but under different conditions might have. The “If” clause of a hypothetical conditional sentence is usually written in the subjunctive mood (see Grammar Corner, VOICE & MOOD). The conditional clause of the sentence usually contains modals such as “could” or “would”:
“If Henry were stronger, he could lift that box.”
“If I were as sick as you, I would go see a doctor.”
“If I had more time, I could get more work done.”