When do you use 是 (shì) in Mandarin?

Use 是 to link two nouns or noun phrases, where English would use 'is/am/are' between a subject and a noun: 我是学生 (wǒ shì xué sheng, I am a student), 他是英国人 (tā shì Yīng guó rén, he is English). The test is simple - if both sides of 'to be' are nouns, you need 是. If the thing after 'to be' is an adjective (cold, tall, tired), you do NOT use 是; the adjective is its own verb.

Why is 我是冷 wrong for 'I am cold'?

Because 冷 (lěng, cold) is not an adjective in the English sense - it is a stative verb that already means 'to be cold'. Putting 是 in front of it doubles up the 'to be' and is ungrammatical. The correct sentence is 我很冷 (wǒ hěn lěng), where 很 is a neutral filler, not the word 'very'. 是 is only for noun-to-noun links.

How do you make 是 negative and into a question?

Negate 是 with 不: 我不是老师 (wǒ bú shì lǎo shī, I am not a teacher). For a yes/no question, add 吗 to the statement: 你是学生吗 (nǐ shì xué sheng ma, are you a student?). For the A-not-A question, repeat the verb around 不: 他是不是中国人 (tā shì bu shì Zhōng guó rén, is he Chinese or not?). The word order never changes.

The Mandarin Copula 是 (shì): To Be, and When to Leave It Out

是 links a noun to a noun

The whole job of 是 is to sit between two nouns and say they are the same thing, or that one is a member of the other.

我是学生。 (Wǒ shì xué sheng.) - I am a student.
他是英国人。 (Tā shì Yīng guó rén.) - He is English.
这是我的书。 (Zhè shì wǒ de shū.) - This is my book.
北京是中国的首都。 (Běi jīng shì Zhōng guó de shǒu dū.) - Beijing is the capital of China.

In every one of these, both sides of 是 are nouns: I / a student, he / an English person, this / my book. That is the entire pattern. If you can rewrite the English as "X is a Y" where Y is a thing, you need 是.

The trap: no 是 before an adjective

This is the mistake every English speaker makes in week one. "I am cold", "she is tired", "Chinese is hard" all use "is" in English, so the instinct is to reach for 是. Do not.

我很冷。 (Wǒ hěn lěng.) - I am cold. NOT 我是冷.
她很累。 (Tā hěn lèi.) - She is tired. NOT 她是累.
中文很难。 (Zhōng wén hěn nán.) - Chinese is hard. NOT 中文是难.

Mandarin adjectives are stative verbs: 冷 already means "to be cold", so 是 has no work to do and inserting it is ungrammatical. The neutral linker here is 很 (hěn), which in this slot is an empty filler, not the word "very". The full story is on the adjectives as stative verbs page.

Negation: 不是

是 negates with 不 (bù), giving 不是 (bú shì, "is not"). Because 是 is fourth tone, the 不 in front shifts to second tone (bú) by tone sandhi.

我不是老师。 (Wǒ bú shì lǎo shī.) - I am not a teacher.
他不是英国人。 (Tā bú shì Yīng guó rén.) - He is not English.
这不是我的。 (Zhè bú shì wǒ de.) - This is not mine.

是 is always negated with 不, never with 没. (没 is reserved for 有 and for completed actions - see the 有 possession page.)

Questions: 吗 and A-not-A

Mandarin questions keep the statement's word order. Two ways to question a 是 sentence:

吗 (ma) at the end turns a statement into a yes/no question: 你是学生吗? (Nǐ shì xué sheng ma?) - Are you a student?
A-not-A repeats the verb around 不: 他是不是中国人? (Tā shì bu shì Zhōng guó rén?) - Is he Chinese (or not)?

Both ask the same thing. The A-not-A form sounds slightly more direct and is common in speech.

When Mandarin drops 是 entirely

For age, dates, days, clock times and prices, Mandarin usually omits the copula. The number or time phrase acts as the predicate on its own.

我二十岁。 (Wǒ èr shí suì.) - I am twenty (years old). NOT 我是二十岁.
今天星期一。 (Jīn tiān xīng qī yī.) - Today is Monday.
现在三点。 (Xiàn zài sān diǎn.) - It is three o'clock now.
这个十块钱。 (Zhè ge shí kuài qián.) - This is ten yuan.

You can add 是 back for emphasis or contrast (今天是星期一,不是星期二), but the bare form is the everyday default.

What to internalise

是 links noun to noun. 我是学生, 他是英国人.
Never 是 before an adjective. 我很冷, not 我是冷.
Negate with 不是, never 没.
吗 or A-not-A for questions, with no change in word order.
Drop 是 for age, dates, times and prices. 我二十岁, 今天星期一.

For the rest of the grammar inventory, see the Mandarin grammar cheatsheet.