Why/when is acceptable to drop the plural+classifier?
You can do this for things you usually buy/sell in bulk such as books, notebooks, pens, oranges, apples, etc.
So is the general rule that if I use any numerical indicator (một, cac, etc.) you must include the classifier?
why you don't use plural form in the vietnamese translation ?
"Tôi mua các cuốn sách" is accepted too.