"Do you eat fish or chicken?"
Translation:Bạn ăn cá hay gà?
Well, "các" has NOTHING TO DO with "do" in this case. "Các" or "những" is used before a SINGULAR NOUN to turn it into a PLURAL NOUN. "Các" can go with "bạn" (singular form of "you") to form "các bạn" (plural form of "you"). More examples:
con gà (chicken);
MỘT con gà (A/ONE chicken);
CÁC/NHỮNG con gà (chickenS).
"Do", on the other hand, when being a VERB means "làm". "Do", however as an AUXILIARY, doesn't have any true meaning in Vietnamese but to DISTINGUISH between TENSES. English speakers make questions using "do" for simple present tense, "did" for simple past and "will/shall" for simple future then we Vietnamese use those hints to get the idea and TRANSLATE the questions into appropriate tenses in our language. Hope this helps you :)
Sorry if my past answer confused you. It's really difficult to explain but I'll try to make it clearer this time:
The verb 'to do' [in English] generally means 'làm' [in Vietnamese]. For examples:
DO as they say! -> Hãy LÀM theo lời họ nói!
I didn't DO my homework. -> Tôi đã không LÀM bài tập về nhà (của tôi).
When will you DO that? -> Khi nào bạn LÀM điều đó?
When used as an auxiliary verb, 'do' doesn't mean 'làm' [in Vietnamese]. You translate auxiliary verbs depending on their roles in sentences. For examples:
DON'T do as they say! -> ĐỪNG làm theo lời họ nói!
I DIDN'T do my homework. -> Tôi ĐÃ KHÔNG làm bài tập về nhà (của tôi).
DO you like oranges? -> Bạn CÓ thích cam KHÔNG?
DOES she love Tom or Bill? -> Cô ấy yêu Tom hay Bill?
I guess Duolingo can't display hints for each part of speech, hence the confusion.