Structure of 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 done it if you'd (you had) known earlier?

Third conditional - common mistakes
Common mistakes Correct version Why?
If I would have asked him, he would have helped me. If I had asked him, he would have helped me. In the if-clause we use the past perfect (had + past participle). We never use 'would' or 'would have' in the if-clause.
If you had spoken to my mother, she would tell you where I was. If you had spoken to my mother, she would have told you where I was. The main clause has
would + have + past participle.