Translation:Do you use the Vietnamese dictionary?
I'm still a little confused as to when to use "có" in a question sentence or not. Any tips?
In yes-no questions similar to this one, you can always use "có...không?". Questions without "có" are still right but they're not as natural as ones that have "có". In daily conversations, people usually use the full structure "có...không?" in their speech.
I think motto of Duolingo is: "Consistency is the hobgoblin of little minds." I am totally confused using "a" and "the", usually get it wrong so this time I don't think and throw one in there and it accepts both. Relying solely on Duolingo is a mistake of course.