Type	Form	Use
Zero	If + present, present	Facts: If you heat ice, it melts.
First	If + present, will + verb	Real future possibility: If it rains, we'll stay in.
Second	If + past, would + verb	Unreal now/future: If I had more time, I'd volunteer.
Third	If + past perfect, would have + pp	Imagined past: If I'd known, I would have called.
Mixed	If + past perfect, would + verb	Past cause, present result: If I'd studied medicine, I'd be a doctor now.

unless = if… not · as long as / provided that = only if · If I were you… for advice.

Never use will or would in the if-clause: ~~If it will rain…~~