Learn French: Elles sont descendues dans la vallée pour le départ. - They went down into the valley for the departure.

This sentence is in the past tense (passé composé) and uses the helping verb être (sont). When a verb uses être, the past participle has to match the subject. Because the subject Elles (they) is feminine and plural, we add an -e for feminine and an -s for plural to the base word descendu, making it descendues.

Why do we use "sont" instead of "ont" to say "they went down"?

Most French verbs use avoir for the past tense, but descendre (to go down) is one of the specific verbs of motion and state change that use être instead. You might have learned these as the DR MRS VANDERTRAMP verbs.

Doesn't "dans" usually mean "in"? Why is it translated as "into" here?

Exactly, dans means "in" or "inside". But when you pair it with a verb of motion like descendre, it shows movement toward the inside of a place. In English, we naturally say "into" to show this movement, making it the best translation for dans la vallée.

Does "le départ" mean "the department"?

No, they just look similar! Le départ comes from the verb partir (to leave) and means "the departure" or "the start". If you want to say "department", you would use le département.