Learn French: Il s'était passé de son sac de couchage. - He had done without his sleeping bag.

While passer means to pass or spend time, and se passer usually means to happen, adding the preposition de creates the idiomatic expression se passer de. This completely changes the meaning to "to do without" or "to forgo" something.

Why do we use était instead of avait here?

This sentence is in the pluperfect tense (plus-que-parfait), which describes an action completed before another past action. Because se passer de is a pronominal verb (indicated by the reflexive pronoun se), it must be conjugated with the auxiliary verb être in all compound tenses. Therefore, we use the imperfect form of être to get s'était.

If we wanted to replace de son sac de couchage to say "He had done without it," which pronoun would we use?

You would use the pronoun en. Because the idiomatic verb phrase se passer de requires the preposition de, the entire object phrase introduced by it is replaced by en. The sentence would become Il s'en était passé.

Would the spelling of passé change if the subject were feminine?