Most explanations of the French past tenses fail for the same reason: they
describe the events instead of the speaker. Try this question instead.
Am I moving the story forward, or painting the scene it
happens in?
Moving forward is passé composé. Painting the scene is
imparfait.
Watch it work
Il pleuvait. Je marchais vers la gare quand mon téléphone a sonné.
The rain and the walking are the scene — the camera is just showing you the
world. The phone ringing is the event that moves the story. Two scene verbs in
imparfait, one story verb in passé composé.
The three cases that trip people up
1. Repeated actions
Quand j’étais petit, j’allais à la mer chaque été. A habit is scenery,
not an event, so it takes imparfait — however many times it happened.
2. Verbs that change meaning
Je savais means “I knew (already)”. J’ai su means “I found
out” — the moment of learning is an event. Same with
pouvoir: je pouvais is “I was able to”, j’ai pu is “I
managed to”.
3. Descriptions of feeling
J’avais peur is a state you were in. J’ai eu peur is the jolt
of fear at a specific moment. Both are correct; they describe different things.
Practise it properly
Tell a two-minute story out loud about the last time something went wrong on
a journey. Record yourself. Then listen back and, for every verb, ask the
question at the top of this article. You will hear your own mistakes faster than
any worksheet will show you.
Want someone to mark that recording? That is exactly what happens in a
private lesson.