The verb 是 (to be): A 是 B
【一36】“是”字句
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
是 (shì) links two nouns: “A is B”. 我是学生 = “I am a student”. To negate, put 不 in front: 我不是老师 (mind the tone: bú shì).
Careful: 是 is much narrower than “to be”. It identifies or classifies (who/what A is), it does not go with adjectives. “I'm busy” is 我很忙, with no 是 (see 很 later).
It can also say what is located somewhere: 车站东边是一个学校 (“east of the station is a school”).
And describe a feature: 这个包是新的 (“this bag is new”, literally “is a new one”).
Examples
我是学生。
Wǒ shì xuésheng.I'm a student.
他不是老师。
Tā bú shì lǎoshī.He isn't a teacher.
这是我的朋友。
Zhè shì wǒ de péngyou.This is my friend.
她是医生吗？
Tā shì yīshēng ma?Is she a doctor?
车站东边是一个学校。
Chēzhàn dōngbian shì yí ge xuéxiào.East of the station is a school.
Common mistakes
✗ 我是很忙 → ✓ 我很忙 (no 是 before adjectives).
✗ 我是去学校 → ✓ 我去学校 (是 is not an auxiliary).
Tone: 不是 is read bú shì, not bù shì.