"Would" is used in Conditional 2 sentences and it is very important to notice that Conditional 2 does not speak about the past, it speaks hypothetically about the present and future.
"Would" is never used in the Condition Clause - the one starting with "if"/"unless." It is always in the Result Clause.
Conditional 2 refers to hypothetical or unreal situations in the present and future (not in the past! - Conditional 3 speaks about the past). What is also worth remembering is that we never use "was" in the Condition Clause. Instead we use "were."

e.g. If John were rich, he would travel around the world (AT PRESENT).
This sentence speaks about the present - John is not rich and he cannot travel around the world NOW.

If we want to construct a Conditional sentence speaking about the past, we should use Conditional 3.

It consists of If + Past Perfect, would + have + 3rd form of the verb

e.g. If John had won the lottery (last year), he would have bought a yacht.

Another thing which is also worth remembering is that in the GMAT we use "if" ONLY in Conditionals.

Cheers,
Kasia