i) Third Second Mixed ConditionalsIf + Past Perfect, + Would + Base Form
1/ For imaginary present actions or situations that are not possible because the necessary conditions were not met in the past. eg: If you had taken the course, you would know about it. (The conditions were not met because the person did not do the course and as a result does not know about it now.)
ii) Second Third Mixed ConditionalsIf + Past Simple, + Would have + Past Participle
1/ To avoid the illogicality of saying 'If I had been you', which means that I was not you on that occasion, but could be in the future, which is, of course, impossible.
eg:If I were you, I wouldn't have done that.
2/ Where the first part is still true:
eg:If I could speak Spanish, I wouldn't have needed to get the letter translated.
This means that I couldn't speak Spanish then when I needed the translator and still can't.