Wrong: Bon matin
Right: Bonjour
Explanation: In English, "good morning" is a typical greeting. Its literal French translation, "bon matin," is not. If you want to greet someone in the morning, you just say bonjour.* The only time you would use bon matin is in the expression de bon matin, which means "bright and early."
Likewise for "good afternoon" - the French simply don't use "bon après-midi" as a greeting. If you want to greet someone in the afternoon, you should again say bonjour or, starting around 6pm, bonsoir. Upon leaving, you can say bon après-midi or bonne soirée to wish someone a nice afternoon or evening, respectively.
*In Québec, bon matin can be used as an informal greeting between, for example, close friends and long-time co-workers.
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