"We order a plate of rice."
Translation:Chúng tôi gọi một đĩa cơm.
As I've explained in many sentences before, when you mention a noun with its quantity, you have to use the corresponding classifier for that noun (the formula: number + classifier + noun, for example: hai con mèo - two cats, ba quả táo - three apples, một cuốn sách - a/one book, bảy cái đĩa - seven plates).
But here is an exceptional situation. When many nouns stand together to form a noun phrase in Vietnamese, you need to know which noun is the main and which nouns are modifiers for the main noun. For example: noun phrases đĩa cơm, ly trà, kệ sách contain a container (đĩa - plate, ly - glass/cup, kệ - shelf) and a contained material (gạo - rice, trà - tea, sách - book) in each one.
Let's pick đĩa cơm for a deeper analysis.
- When the container đĩa is the main noun (you concern about the container more than the contained material), the whole "đĩa cơm" will mean "plates for eating rice/plates that are used to eat rice" (people also call them "đĩa ăn cơm"). Then you should use đĩa's corresponding classifiers (cái/chiếc) when the quantity of the noun phrase is mentioned.
một cái đĩa (ăn) cơm - a rice plate/a plate that is used to eat rice.
This also works for other similar noun phrases.
ba cái ly/tách trà - three tea cups
năm cái kệ sách - five book shelves
- When the contained material cơm is the main noun (you concern about the contained material more than the container), the whole "đĩa cơm" will mean "the rice dish that is served in a plate". If there is a quantity indicator (such as numbers, plural indicators (những, các)) standing before the noun phrase, this quantity indicator will show the quantity of the container (similar to English) and we don't use classifiers in this case.
một đĩa cơm - a plate of rice
mười ly trà - ten cups/glasses of tea
hai kệ sách - two shelves of books (NOT "shelf of two books")
You may wonder that why the second situation isn't like the first. Why don't we use classifiers for the contained materials in the second situation like we do to the containers in the first situation? The answer is that even when the contained materials are the main nouns in those phrases, their exact quantities are still unknown. So we don't use classifiers for them.