Conditional sentences are structures we use to hypothesize about a possible outcome of a present action, e.g., If you leave before the credits you will miss the final scene (so you’d better stay until the very end of the movie), an impossible outcome of a present action, e.g., If you left before the credits you would miss the final scene (but, since you’re not planning on leaving early, you will not miss the ending) or an impossible outcome of a past action, e.g., If you had left before the credits you would have missed the final scene (but you stayed and so you got to see the final scene).

A FORM

There are four basic types of conditional sentences:

§ the zero conditional: if + Present simple + Present simple

§ the first conditional: if + Present (simple/continuous/perfect) + will/may/might/must/should + bare infinitive

§ the second conditional: if + Past (simple/continuous) + would/could/might + bare infinitive

§ the third conditional: if + Past perfect + would/might/could have + past participle

B use

The zero conditional, if + Present simple + Present simple, is used in situations, in which the outcome logically (nearly automatically) follows from the cause, e.g.:

§ If you drink a lot of coffee, the level of magnesium in your body drops.

§ If a dog feels threatened, it bites.

§ If you don’t get enough sleep, your immune system suffers.

§ Water starts boiling if you heat it long enough.

Note: We only separate the two clauses with a comma if the if-clause comes first.