Try removing the definite article "the" and see if the phrase makes any sense to you? By the way, kids don't swear! Please ask questions politely!
Bạn thử bỏ mạo từ xác định "the" và xem cụm từ này có nghĩa phù hợp không? Ngoài ra, trẻ em không nên nói tục và khi đặt câu hỏi thì phải lịch sự một chút!
Why though? We don't know whether fish are normally considered dirty or not. A dirty fish vs The dirty fish to me depends on whether the translator wants to call this fish out specifically, and the article itself does not appear to have any clues to pull from the base Vietnamese to guide this interpretation.
.... I have to wonder if the single dirty fish in a crowd of clean fish would be called out as MỘT con cá dơ / The (single) dirty fish.
Since Vietnamese doesn't have articles as English does, I see the difficulty in translating from one language to another.
As a native Vietnamese, I see 'con cá dơ' only as 'THE dirty fish' (the only fish that is dirty among other not dirty ones). 'THE dirty fish' is definitely 'A dirty fish' (MỘT con cá dơ) but A specific one. Vietnamese people don't say or write MỘT in this case; MỘT is implied. If we are to say 'MỘT con cá dơ', we mean 'A dirty fish' not 'THE dirty fish'. 'MỘT con cá dơ' is 'A dirty fish' in general, not a specific one.
It may add to the confusion since FISH can be both singular and plural in English. Here I give you some simple examples:
Cô gái đẹp -> THE beautiful girl [in specific]
MỘT cô gái đẹp -> A beautiful girl [in general]
NHỮNG/CÁC cô gái đẹp -> (THE) beautiful girlS [in general or in specific depends on its context]
Gái đẹp -> Beautiful girlS [in general]
I do agree that we don't really know the context of 'con cá dơ'. However, in order not to confuse yourself and the people with whom you're talking:
- Add MỘT when you mean A/AN (chicken: con gà -> A chicken: MỘT con gà)
- Add A/AN when you mean MỘT (con gà: chicken -> MỘT con gà: A chicken)
- DON'T add MỘT when you mean THE (chicken: con gà -> THE chicken: con gà -> THE chickenS: NHỮNG/CÁC con gà)
There are always exceptions, but we will discuss them later on. (I'm out of breath) :P