Conditionals

Conditional sentences contain a conditional clause (introduced by words such as if, as long as and unless) and a main clause.
If the conditional clause comes before the main clause, a comma is needed (as in this sentence).
A comma is not needed if the conditional clause comes after the main clause (as in this sentence).

Zero Conditional
if + present simple or imperative
We use the zero conditional to talk about situations which are always true. If has the same meaning as when, whenever or every time in such sentences.
Everyday situations: My eyes hurt if I spend too long on the laptop.
Scientific facts: If you mix blue and yellow, you get green.
Instructions (if+imperative): If you don't know the answer, make an intelligent guess.

First Conditional
if + present simple, will + infinitive without to
We use the first conditional to talk about possible situations and their probable results in the future.
She'll be very happy if you phone her.
It can be used for warning, promises and threats.
I'll send you to bed if you don't behave yourself.
Modal verbs can be used in the main clause in place of will. May, might and could, for example, introduce possible results if a conditional is met.
If I finish my homework early, I might call you.

Second Conditional
if + past simple, would + infinitive without to
We use the second conditional to talk about imaginary, unlikely or impossible situations in the present or future.
If I knew the answer to number six, I would tell you.
First or second conditional? Notice the difference in meaning between these two sentences:
1st Cond. If they give me a pay rise, I'll buy a new car. (I feel there is a real possibility that they will give me a pay rise.)
2nd Cond. If they gave me a pay rise, I'd buy a new car. (I feel it is less likely that they will give me a pray rise.)
The second conditional can also be used to give advice.
If I were you, I'd complain to the manager.
Both was and were are possible in the conditional clause after the subject pronouns I/he/she/it. Was is more common in spoken English.
If he were a little taller, he'd be an excellent goalkeeper.
The modals verbs might and could can be used in the main clause to indicate possible results.
If you worked a bit harder you might have more success.

Third Conditional
if + past perfect, would/might/could have + past participle
We use the third conditional to talk about imaginary situations in the past.
If we hadn't taken a map, we would have got lost. (we took a map, so we didn't get lost.)

Mixed Conditional
if + past perfect, would + infinitive without to
Mixed conditionals are a combination of a second and a third conditional. They can express an imaginary past event and a possible or probable present result.
If you'd listened to my advice, you would not be in this situation now.

Alternative words for if
As long as, provided (that), providing (that) and on condition (that) can be used in place of if to emphasize the condition.
I'll lend you 10€ as long as you give it back tomorrow.