Which classifier: cái for things, con for animals, người for people, and the common others

Before most counted or particular nouns, Vietnamese puts a classifier, a short counting word that says what kind of thing it is: hai con mèo (two cats), cái bàn này (this table). Which one a noun takes belongs to the word, so learn it with the word. But the system behind it is simple at the core. Words that are units themselves take none: hai ngày (two days), ba tuần (three weeks), một lần (once).

Cái is for things: cái ghế, cái áo, cái điện thoại. It's the best guess for an object. Con is for animals, con chó, con cá, and, by habit, for a few things like roads, rivers and knives: con đường, con sông, con dao. Người is for people: ba người bạn; vị is the respectful version, and đứa is the everyday word for children: hai đứa con (two kids).

A few more sort things by shape or kind: cây for long, thin things (cây bút), tờ for sheets of paper, trái for fruit and round things (trái cam; in Hà Nội quả), cuốn for books (Hà Nội also quyển). Chiếc overlaps with cái, especially for vehicles and one of a pair: chiếc xe, chiếc đũa. Đôi counts pairs: một đôi giày. Trousers and scissors are one thing: cái quần, cái kéo.

Thằng, for a boy or a man, is either very casual or looks down on him, so avoid it with adults. When you're unsure, cái for a thing is rarely far off.