Conditional Sentences are also known as Conditional Clauses or If Clauses. They are used to express that the action in the main clause (without if) can only take place if a certain condition (in the clause with if) is fulfilled.

The zero conditional is a structure used for talking about general truths
A zero conditional sentence consists of two clauses, an “if” clause and a main clause (In most zero conditional sentences you can use when or if and the meaning will stay the same.):
If you heat water to 100 degrees,        it boils.
If the “if” clause comes first, a comma is usually used. If the “if” clause comes second, there is no need for a comma:
Water boils        if you heat it to 100 degrees
If it rains, the grass gets wet.        This is basically always true — the rain makes the grass wet.

Conditional 1 :The first conditional is used to talk about things which are possible in the present or the future — things which may happen:
If you study hard,        you will pass the test.
If it's sunny, we'll go to the park.        Maybe it will be sunny — that's possible.

Conditional 2
Often called the "unreal" conditional because it is used for unreal - impossible or improbable - situations. This conditional provides an imaginary result for a given situation.
NOTE
The verb 'to be', when used in the 2nd conditional, is always conjugated as 'were'.
If he studied more, he would pass the exam.
I would lower taxes if I were the President.

Conditional 3
Often referred to as the "past" conditional because it concerns only past situations with hypothetical results. Used to express a hypothetical result to a past given situation.
If he had known that, he would have decided differently.
Jane would have found a new job if she had stayed in Boston.