Learn French: Il s'est excusé parce qu'il avait oublié la réunion. - He apologized because he had forgotten the meeting.

Avait oublié is the plus-que-parfait. We use this tense to describe an action in the past that happened before another action in the past. Here, he first forgot the meeting, and then later he apologized (il s'est excusé) for it.

How is the plus-que-parfait formed in avait oublié?

It is built just like the passé composé, but the helping verb is in the imparfait tense. Since oublier uses avoir as its helping verb, we take the imparfait form avait (he had) and add the past participle oublié (forgotten).

Why is the first verb s'est excusé using est instead of a?

The verb is s'excuser (to apologize), which is a reflexive verb. As a quick refresher, all reflexive verbs use the helping verb être in compound past tenses rather than avoir. That is why we say il s'est excusé instead of using a form of avoir.

Does s'excuser literally mean to excuse oneself?

Yes! While it translates to apologizing in English, the French literally say he excused himself. Because it is an action you do to yourself, it requires the reflexive pronoun s' before the verb.