Counting things: number + classifier + noun (hai con chó)

To count things, put the number first, then a classifier, then the noun: hai con chó (two dogs), ba cái ghế (three chairs), một cuốn sách (a book). The classifier is a short counting word that most nouns need after a number; hai chó sounds broken. Một (one) often does the work of "a": Em muốn mua một cái áo.

The noun itself never changes for the plural, and no plural word is added: the number already says how many, so ba con chó.

Some nouns are counted directly, with no classifier. Many are units in themselves: người (person), ngày (day), năm (year), tuổi (year of age), lần (time). So ba người, hai ngày. Words for jobs, and for places such as schools and hospitals, also go without: hai bác sĩ, một bệnh viện.

Which classifier a noun takes, cái for things, con for animals and so on, has its own page. Mấy (how many) takes the same classifier as a number: mấy con mèo. For things you measure rather than count, like water or rice, use a container: hai chai nước.