How many, how much: bao nhiêu

Bao nhiêu asks how many or how much, with no guess about the size. The answer can be any number: forty students, a few hundred thousand for a shirt, two kilos of rice. It also works straight on things you don't count one by one, such as money, rice or sugar; mấy needs a unit there: mấy ký gạo, mấy chục ngàn.

It goes where the number will go in the answer: Lớp em có bao nhiêu học sinh? Lớp em có bốn mươi học sinh. A counting word can follow, as after a number, but with bao nhiêu it's often left out.

For prices, bao nhiêu or bao nhiêu tiền comes after the thing: Cái áo này bao nhiêu? At the market you'll also hear the price per unit: Xoài bao nhiêu một ký? In Hà Nội a kilo is a cân.

It's the polite way to ask an adult's age, too: Anh bao nhiêu tuổi? For a child's age, or any small number you expect, mấy is more natural.

Two questions look similar but ask other things. Bao lâu is how long: Anh ở đây bao lâu? Bao giờ is when, though in Saigon khi nào is more common.