There are three possible constructions:
(1) REALITY   Present Simple will + I
(2) POSSIBILITY     Past Simple  would + I
(3) IMPOSSIBILITY Past Perfectwould + have + III
Pay attention to the different meaning of the following sentences:
TYPE 1: If I have the chance, I will go abroad.
The first sentence indicates that it is likely that I have the chance to go abroad.
TYPE 2: If I had the chance, I would go abroad.
According to the second sentence, it is less likely but still possible.
TYPE 3: If I had had the chance, I would have gone abroad.
The third sentence indicates that the possibility is no longer given; the statement refers to an option in the past.
Additional to these three types, we can also use the present simple tense in both parts of the sentence. This type is also called the Zero Conditional and is used when the result will always happen.
e.g. If you heat water, it boils.   > It is a fact.