What is imparfait and passé composé and what are some examples of each?

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Oh well. French tenses are quite complicated. I'll try my best:
Imparfait is generally used for descriptions, and habits, or when the action is in the past but not quite finished.
E.g. Il était souriant dès le matin. (Descriptive)
Il prenait toujours sa voiture. (Habit)

Passé Composé is complementary to that, in the sense that it is used for precise and finished actions.
Examples:
Elle a traversé la rue. (Once the road is crossed, the action is finished).

Both tenses so that it's clearer:
"Jean voulait me parler mais je suis parti avant."
"Je fumais beaucoup mais j'ai arrêté"