If-clauses for real, unreal and impossible situations — from everyday facts to imagined pasts.
If + present, present — facts and things that are always true.
If + present, will — real possibilities in the future.
If + past, would — unreal or unlikely situations in the present or future.
If + past perfect, would have — imagining a different past.
Unless, as long as, provided that — other ways to express a condition.
Mixing time frames — a past condition with a present result, or the reverse.