Number + measure word + noun
Mandarin almost never lets a number sit directly next to a noun. Between them you need a measure word (also called a 'classifier') that tells you HOW you're counting the thing. The pattern is rigid: NUM + MW + NOUN. 三个学生 = three students, 两本书 = two books, 一只狗 = one dog. The measure word depends on the shape, type, or category of the noun. There are dozens, but five or six get you through everyday speech — and 个 is the safe default when you forget which one to use.
个 — the universal counter
个 gè is the workhorse. It counts people, abstract things, and most generic nouns: 一个人 (one person), 三个朋友 (three friends), 五个学生 (five students), 一个名字 (a name), 两个问题 (two questions). When in doubt, 个 is almost always understood, even when a more specific word would be more elegant. In quick speech the tone often weakens to neutral: yí ge, liǎng ge, sān ge.
二 vs. 两 — the 'two' problem
Mandarin has two words for 'two'. 二 èr is the cardinal number — what you say when reciting digits, reading addresses, or doing math: 一,二,三. But when you mean 'two of something', you switch to 两 liǎng: 两个人 (two people), 两本书 (two books), 两只猫 (two cats). NEVER 二个人. The rule: counting → 二, measuring → 两. (Exception: in compound numbers like 十二 (twelve) or 二十 (twenty), it stays 二.)
The most common specific measure words
本 běn for bound things — books, magazines, notebooks: 一本书, 三本杂志. 只 zhī for most animals and one of a paired body part: 两只猫, 一只狗, 一只手 (one hand). 张 zhāng for flat things — paper, tickets, tables, beds, photos: 一张纸, 两张票, 一张桌子. 杯 bēi for cup-fuls of drinks: 一杯水, 两杯咖啡, 一杯茶. 块 kuài for chunks/pieces — and crucially for the everyday word for the yuan: 一块钱 (one yuan), 三块蛋糕 (three pieces of cake).
这 / 那 + measure word
Demonstratives use the same slot as numbers. Instead of NUM + MW + NOUN, use 这/那 + MW + NOUN: 这个人 (this person), 那本书 (that book), 这只猫 (this cat), 那张纸 (that piece of paper). For plural demonstratives, add a number or 些: 这三个学生, 那些书 (those books — 些 is its own word for 'some/several').
When you can skip the measure word
A few special nouns ARE their own measure word and don't take one: 岁 (years old) from lesson 5 — you say 十岁, never 十个岁. Same for 年 (year), 天 (day), 次 (time/occurrence). These are 'unit nouns' that already carry the counting built in.
