Use ah bon ? when you want to:
It is such a simple comment but it can go a long way. The intonation of your voice will indicate if you’re expressing surprise, acknowledgment or understanding.
- indicate surprise
- indicate an understanding of what was said
- indicate an interest in what was said
- show a desire to want to hear more of the conversation
- show a desire to contribute to the conversation
Don't be fooled by bon, it doesn't mean "good" here. So ah bon ? doesn’t mean “oh good” and it can be used when talking about both good and bad things.
ExamplesTu ne devineras jamais ! Il se marie demain. - Ah bon ?
You’ll never guess! He’s getting married tomorrow. - No way!
J'ai vu un film intéressant hier. - Ah bon ?
I saw an interesting movie yesterday. - Oh, yeah? You did?
Ils ont déménagé après le décès de leur père. - Ah bon ?
They moved after their father died. - I see.