Hi Nghia,

"En 2004, j'avais de l'argent" --> "During the period of 2004 I had some money"

This is an imparfait situation.     Why?    What is being described here is an historic situation that was the case during the duration of that past period. It is not a completed action, it was an enduring status then, but being reported on now.

This is how I see it  --  hope it helps.

Bonne continuation.

Jim

There are so-called verbes d'état (verbs of state) in French. They describe states of being and, as such, are almost always used in the imperfect tense instead of the passé composé. Here is a link where Cécile explained the concept: https://progress.lawlessfrench.com/questions/view/what-are-verbes-d-etat-s-il-vous-plait