Third conditional: when and how to use

How to form the third conditional

Structure of third conditional
positive	negative	question
If I'd (I had) known you were coming, I would have waited for you.	If she hadn't (had not) been ill, she would have gone to the cinema.	Would you have told me if you'd (you had) known about it?

We use the third conditional to express a theoretical past situation and its theoretical result in the past:

The third conditional – common mistakes
Common mistakes	Correct version	Why?
if I would have asked	If I had asked him, he would have helped me.	In the if-clause we use the past perfect (had + past participle). We don't use would or would have in the if-clause.
she would tell you	If you had spoken to my mother, she would have told you where I was.	The structure of the main clause is would + have + past participle.