Translate the following: If I had been listening to my sister, I would have understood why she was late.

This is a hypothetical, or si clause, specifically the third type: si + pluperfect + conditional perfect. If I had been listening to my sister - this is the pluperfect, a tense further in the past. It’s formed by taking the imperfect form of the auxiliary verbs avoir or être (depending on the verb) and the past participle of the verb in the sentence. Here: si j’avais écouté ma sœur Note: the verbal construction is écouter qqn, not écouter à. I would have understood: Here is the second part, the conditional perfect. It is formed in the same way as the pluperfect, except that the auxiliary is now in the conditional. Here: j’aurais compris Why she was late: there are a few options:Pourquoi elle était (imperfect, as it’s describing a situation) en retard However, I prefer la raison pour laquelle elle était en retard.So the full sentence is: Si j'avais écouté ma soeur, j'aurais compris la raison pour laquelle elle était en retard.