You already have one past tense, the passé composé, for things that happened and were over: Hier, j'ai mangé au restaurant. French has a second one, the imparfait — the past of what used to happen, again and again, or of what was going on for a while:

• Le mercredi, je jouais au foot avec mes copains. — On Wednesdays, I used to play football with my friends.
• Le dimanche, nous mangions chez ma grand-mère. — On Sundays, we'd eat at my grandmother's.
• À l'époque, elle habitait à Lyon. — At the time, she lived in Lyon.

English has three ways to describe a habit like this — I used to play, I would play, or a plain I played — and French uses this one tense for all three.

One form to learn straight away: j'étais is être in this tense. Quand j'étais petit (or petite) — when I was little — is how many French memories begin.