I was told by a reliable source that "J'aurais dû partir plus tôt" means "I should have left sooner." So how do you say, "I must have left sooner."? Or "I would have left sooner."
The question here is about I "should have", vs “would have”, vs “must have”. The “left sooner” could be any verb. e.g. “I should have purchased”, “I must have purchased”, “I would have purchased” are the same question.