This is used when there is no condition; in other words you could substitute 'if' with 'when'. It is often used when describing facts or explaining how something works.
Structure: if + present • present
- If you don't water flowers, they die.
- If you turn that switch to the black position, the power gets cut off.
- If you mix water with oil, the oil floats.
The first conditional is used for situations based on facts. The condition describes something normal and possible, and the result is probable and based on the present or the future.
Structure: if + (any) present • future/imperative/modal verb
- If you study hard, you will pass your exams.
- If you click on that icon, you'll lose anything you haven't saved.
- If it doesn't rain tomorrow, we're going to the beach.
- If you've finished, you may go.
- If you've finished , close your books!
- If it's raining, don't go out!
This conditional is not based on facts. It refers to a situation in the present or future which is unreal, unlikely or contrary to facts. To show this unreality, we have to shift the tense from the present to the past, although the condition still refers to the present or the future.
Structure: if + past • would (or could, or might) + infinitive
- If I won the lottery, I would buy a fast car.
- If I had some money, I would/might give you some ("but I haven't got any money, so you can't have any").
- If I were(1) you, I wouldn't do it ("I'm not you, it's a piece of advice").
- If that car were cheaper, I would buy it. (Not "If that car was cheaper...").
(1) In unreal conditionals the form "was" is not considered grammatically correct. In written English or in testing situations, you should always use "were." However, in everyday conversation, "was" is often used.
The third conditional refers to situations in the past which, because they're in the past, are imaginary or impossible. You can't change the past.
Structure: if + past perfect • would (or could, or might) have + past participle
- If I had studied more, I would have passed my exams ("but I went out every night with my friends, didn't open a book, and I failed").
- If I hadn't spent all my money on CDs I could have given some to you ("but I did spend it on CDs and I wouldn't give you any money anyway").
- If you had been ready on time, we wouldn't have missed the train and we would have arrived before all the restaurants closed ("but you were too slow and now we're hungry and there's nowhere open").