Form and rules
How it works
Second conditional: if + past simple, would/could/might + base verb for hypothetical present/future meaning.
Third conditional: if + past perfect, would/could/might have + past participle for unreal past meaning.
The time reference determines the form: Is the imagined change about now/future, or about a completed past event?
Use were in formal/careful hypothetical be clauses, especially If I were you.
Do not put would in a standard if-clause; use the required past or past-perfect form.