In short

是 (shì) is the Chinese “to be” that equates two nouns: A 是 B means “A is B.” Put it between the subject and a noun that names or classifies it (我是学生 - “I am a student”), and negate it with 不是. Remember one thing above all: 是 links nouns, never adjectives.

The rule

是 (shì) is the equative verb “to be.” It stands between two noun phrases and says that the first one is - or belongs to the category of - the second. This is the sentence pattern Chinese grammar calls the 「是」字句 (shì zì jù, “是 sentence”).

Pattern: Subject + 是 + noun phrase.

• Step 1 - Start with a noun or pronoun as the subject: 我 (wǒ, “I”), 他 (tā, “he”), 这 (zhè, “this”).
• Step 2 - Add 是 (shì).
• Step 3 - Finish with the noun phrase that identifies or classifies the subject - a role, a nationality, a name, or a kind of thing.

我是学生。 (Wǒ shì xuéshēng.) - “I am a student.”

The noun phrase after 是 can be a bare noun (学生, “student”), a nationality (中国人, “Chinese person”), a name or title (王老师, “Teacher Wang”), or a possessive with 的 (我的书, “my book”).

Four facts do most of the work:

• No conjugation. 是 is identical for every person, number, and time. Chinese verbs never change shape, so 我是 / 你是 / 他们是 are all just 是. Whether the meaning is past, present, or future comes from context or a time word, not from the verb.
• 是 links NOUNS, not adjectives. “I am a student” ends in a noun, so you use 是: 我是学生. But “I am tall” ends in an adjective, which drops 是 entirely and takes 很 (hěn) instead: 我很高 (Wǒ hěn gāo) - never 我是高. That adjective pattern belongs to a later lesson; here, only reach for 是 when a noun follows.
• Negation is 不是. Put 不 (bù) directly before 是: 我不是老师 (Wǒ bú shì lǎoshī, “I am not a teacher”). Never negate 是 with 没 (méi). Note the pronunciation: 不 is toned bù on its own, but before the 4th-tone 是 it shifts up to bú, so 不是 is said bú shì.
• 是 is a linker, not “there is / at.” It equates two nouns; it does not express existence (“there is a book”) or location (“I am at home”). Those use 有 (yǒu) and 在 (zài), covered in their own lesson.

To reply to a 是 question - 他是医生吗？ (Tā shì yīshēng ma?), the statement 他是医生 with the sentence-final 吗 of Yes/no questions with 吗 and sentence-final particles hung on the end - agree with 是 or 是的 (shì de, “Yes / That’s right”) and disagree with 不是 (“No”).

Forms & patterns

是 does not change for the subject - one form fits all:

Affirmative, negative, and short answers:

Examples

Common mistakes

• 我是高。 -> 我很高。 (Wǒ hěn gāo.) - An adjective never takes 是; use 很 (or another degree adverb) with it. 是 links nouns only.
• 我没是学生。 -> 我不是学生。 (Wǒ bú shì xuéshēng.) - Negate 是 with 不, never 没. 没 is for 有 and completed actions.
• 我学生。 -> 我是学生。 - Two nouns must be joined by 是. Do not model this on the adjective pattern (我很高), which is the one that has no verb.
• 我是家。 -> 我在家。 (Wǒ zài jiā.) - “To be at a place” is 在, not 是. 是 equates nouns; it does not mark location.

Tips

• No conjugation, ever. 是 is the same for 我 / 你 / 他 / 我们 and for past, present, and future alike. Learn one word and you have them all.
• “A thing” -> 是, “a quality” -> 很. If the word after the subject is a noun (a student, a doctor, Chinese), use 是; if it is an adjective (tall, tired, good), drop 是 and use 很. This is the single most common 是 error for English speakers.
• Say 不是 as bú shì. 不 is toned bù by itself, but before the 4th-tone 是 it rises to bú. The character is written the same; only the tone shifts.
• Two different 是. The 是 here just equates two nouns. A separate structure, 是……的, uses 是 to spotlight when, where, or how something happened - do not mix the two up.