While the lesson is clear that the le passé composé is used here to use depuis for negation, I'm not sure how to ask the question that leads to this response:
For example: When was the last time you your mother saw you?
- Is it: Quelle était la dèrniere fois/Depuis quand ta mere t'a vu/te vois?
In either the case the answer would be in passé composé: for instance: Elle ne m'a pas vu depuis longtemps
Thanks!