Learn French: Les pompiers sont arrivés parce que c'était une urgence. - The firefighters arrived because it was an emergency.

This sentence shows how the two past tenses work together. sont arrivés (passé composé) is used for a specific, completed action: the arrival. c'était (imparfait) provides the background context or the reason why they arrived: the ongoing state of it being an emergency.

Why does arrivés have an -s at the end?

The verb arriver uses être (to be) as its helper verb in the passé composé. When a verb uses être, its past participle must agree with the subject. Since les pompiers (the firefighters) is masculine plural, we add an -s to make arrivés plural.

Why do we use c'était instead of il était for "it was"?

c'était is the imparfait (past tense) form of c'est (it is). In French, you use c'est or c'était when followed by a noun phrase, like une urgence (an emergency). You would only use il était if it were followed directly by a lone adjective.

I see an apostrophe in c'était. Why doesn't parce que contract to parce qu' here?