个

个 (gè) is the most common measure word in Chinese — the one you use between a number/demonstrative and a noun when no more specific measure word fits. 三个人 'three people,' 这个 'this one,' 那个苹果 'that apple.' Mandarin requires a measure word between a number and a noun (you can't say 三人 in modern speech the way you can say 'three people' in English), and 个 is the default. Two things to know: first, 个 is often pronounced in the neutral tone (ge) in fast speech even though its citation tone is fourth (gè); second, 个 is the universal fallback, but every noun has a 'correct' measure word — 一只猫 'one cat,' 一本书 'one book,' 一杯水 'one cup of water.' Using 一个猫 is comprehensible but marks you as a beginner.

Note: 个 is the safest fallback, but in formal writing or careful speech, learners should use the specific measure word: 一本书 (book), 一只猫 (cat), 一辆车 (car). Over-using 个 sounds lazy in writing.

Memory hook: 个 looks like a single person standing under a roof — one individual, one count, one 'piece.'