Passé composé or imparfait?
Both describe the past. The choice depends on how you present the action, rather than a direct match with an English tense.
An event: passé composé
Hier, j’ai acheté un livre.
Yesterday I bought a book. The purchase is presented as a completed event.
A background or habit: imparfait
Quand j’étais enfant, je lisais beaucoup.
When I was a child, I used to read a lot. The sentence describes a past habit.
Use them together
Quand je suis arrivé, il pleuvait.
When I arrived, it was raining. The arrival is an event; the rain describes the background.
Try it: Hier, nous ___ ce film. (regarder, completed viewing)
Hier, nous avons regardé ce film.
Compare the passé composé and imparfait of lire.