"They buy not only computers but also telephones."
Translation:Họ mua không những máy tính mà còn điện thoại.
"if I use plurals I must use classifiers" - you're right.
E.g: Những quả cam (oranges), not Những cam; Những quyển sách (books), not Những sách.
"if I don't use plurals I can omit them" - it depends on the context.
1. If you want to talk about a specific thing, you should use the classifier (nearly like the way you use "the" in English).
E.g: Tôi muốn con mèo = I want the cat (this cat is a specific cat).
2. If you want to talk about a thing in general (1) or there is already a possessive adjective for that noun and the quantity of that noun is one (2), you can omit the classifier.
E.g: (1) Things in general: Mèo có bốn chân = Con mèo có bốn chân = Cats have four legs.
(2) Bạn có muốn mèo của tôi không? = Bạn có muốn con mèo của tôi không? = Do you want my cat? (Note that there is just one cat)
When do you need the verb in the aecond part of the sentence and when do you not? In this case, I put "mua điện thoại" but that was wrong. In another question, I left out the verb in the second part of the sentence "biết tiếng Anh" and was marked wrong then too. Is there a reason?
OMG! Do you know how many sentences I've been marked wrong for not having the "cai" before the may tinh and the dien thoai?!? SPECIFICALLY before those two!!! And now I got marked wrong FOR having them, when the rest of the sentence was exactly the same. You can't win with the Duo Vietnamese course, I swear!!