Plus-que-parfait

When

Something that had already happened before another moment in the past. Il était parti quand je suis arrivé. (He had left when I arrived.)

How to form it

Avoir or être in the imparfait + past participle. J'avais mangé, elle était partie.

Pitfalls

• The same choice between avoir and être as in the passé composé, and the same agreement of the participle.
• It works almost exactly like English "had done". Where English uses that, French usually does too.