Past Perfect + Would have
Incorrect : If she would invite me, I would have gone there.
Correct : If she had invited me, I would have gone there.
Incorrect : If you took this medicine, you could have died.
Correct : If you had taken this medicine, you could have died.
Note: An ‘impossible’ condition referring to an unrealistic past action (an action which actually did not happen in the past) is expressed by the past perfect tense and in this case the principal clause contains would, could or might followed by the perfect infinitive. The sentence ‘If he had been well, he would have attended the meeting’ means that he was not well and so he did not attend the meeting.