Nói câu này trong Tiếng Anh (Mỹ) như thế nào? why not 'charge the fine' instead of 'change a fine'?

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Yeah, this is basically a question of: when to use the ‘definite article’ (“the”) versus one of the ‘indefinite articles’ (“a”/“an”/“the”)

I’ll explain it a bit, but it might be easier to understand & more comprehensive if you look it up in your native language:

You need to use ‘a/an’ when introducing something for the first time (such as a fine). Once you’ve done that, you can use ‘the’ to refer to it for the rest of the conversation (& all other conversations, as long as the person you’re conversing with knows the specific ‘fine’ you’re talking about).

Ex.
A: I’m having a party on Friday.
B: Oh, cool! Whose coming to the party?

Next Wednesday:
B: How was the party?
A: It was great!