Unlikely or contrary to present fact
FORM: If + past simple/continuous + would/could etc. + past participle in the main cause
USE: To talk about something, to give an advice.
1. To talk about something:
That is contrary to the presents facts, or seen as very unlikely to happen
– If I was/were twenty years younger, I‘d emigrate
– If I was/were Prime Minister, I‘d make health care free.
Which is very unlikely to happen in the future
– No matter how safe it was, I wouldn’t want to travel in space.
– I wouldn’t apply for that job unless I thought I had a good change of getting it.
2. To give an advice:
– I’d write it out again if I were you.
Contrary to fact in the past
FORM: If + past perfect + would/could etc. have + past participle in the main clause.
USE: to describe something in the past that could have happened but didn’t, or that shouln’t have happened but did.
– I wouldn’t have told her that if I‘d known she’d tell everyone else.
– She could have got there on time if she‘d not missed the bus.