It really depends. If you are talking about a specific detail that happened in the past, or an activity you used to do continuously in the past, you use imparfait or to describe a singular event that happened (Last year, last week, last Sunday etc.) you use passe compose avec avoir or etre as your auxiliary verbs.
The following are the verb endings for imparfait -
-ais, ais, ait, ait, ions, iez, aient
To form the passe compose avec avoir/etre you have the pronoun (je, tu, il, elle, nous, vous, ils, elles) followed by the auxiliary present tense of avoir/etre and the past participle.
