How do I know when to use the passé composé or the imparfait?

So the passé composé (simple past) and imparfait (imperfect) might seem very similar since they both describe actions in the past. However there are some important differences. The key thing to remember is that the passé composé describes past actions which are completed, which have been finished (e.g. I ate my sandwich - I ate the sandwich, and then I stopped eating the sandwich once it was finished).

Whereas the imperfect is used for

a) actions you "used to do" regularly in the past (e.g. When I was younger, I used to play football - je jouais au foot)

b) to describe how something was in the past (e.g. It was sunny - il y avait du soleil)

c) an interrupted action: This scenario is slightly more complicated, as the imparfait and passé composé are often used together.

E.g. While I was eating, my mother arrived - Quand je mangeais, ma mère est arrivée. The idea is that during the period of time while I was eating (an ongoing process), my mother suddenly arrived. Since arriving is a quick, and importantly a completed action, we use the passé composé, elle est arrivée.