个【gè】

个 is the measure word for people and for nouns with no measure word of their own.

In speech it is usually toneless, ge rather than gè.

A number cannot touch a noun directly

Sān三xué sheng学生 is wrong. Every counted noun needs a measure word between the number and the noun, and 个 is the one to reach for when nothing more specific applies.

It is not a universal measure word

个 is the default, not a wildcard. Nouns that have their own measure word keep it, and using 个 instead sounds wrong to a listener:

• 书 takes 本: 一本书
• 纸 and 桌子 take 张: 一张纸
• 笔 takes 支: 一支笔
• 衣服 takes 件: 一件衣服
• 茶 takes 杯: 一杯茶

个 with 这, 那 and 哪

Demonstratives need a measure word too, and 个 is again the default.

两个, not 二个

Before a measure word, "two" is 两.