Breakdown of Nous étions tristes parce que nous avions perdu le passeport.
être
to be
nous
we / us (object pronoun)
avoir
to have
le passeport
the passport
Questions & Answers about Nous étions tristes parce que nous avions perdu le passeport.
Why do we use avions perdu instead of the passé composé avons perdu?
Losing the passport happened before the state of being sad. Since being sad is already in the past (étions), we need the plus-que-parfait (avions perdu) to show an action that happened even further back in time. It translates perfectly to the English 'had lost'.
Why is étions in the imperfect tense rather than the passé composé?
We use the imperfect (l'imparfait) here because being sad describes an ongoing state of mind or emotional condition in the past, rather than a single, completed event.
How exactly is the plus-que-parfait built in the phrase nous avions perdu?
It is built just like the passé composé, but you put the helping verb (in this case avoir) into the imperfect tense instead of the present. So nous avons becomes nous avions, and then you just add the past participle perdu.