不是 and 不是…而是 (Correcting) | Chinese (Mandarin) Grammar

Chinese has two negators — 不 (bù) and 没 (méi) — and choosing between them usually takes some thought. With 是, though, there is nothing to think about: 是 is negated only with 不, giving 不是 (bú shì, "is not"), and 没是 is simply never Chinese. This page nails that rule down, explains why it holds so cleanly, and then builds on it with the two most useful correcting frames in the language: 不是…而是… ("not X, but rather Y") and 不是…就是… ("if not X, then Y"). For the general 不-versus-没 decision, see 不 vs 没; here we stay on 是.

是 negates with 不, never 没

The rule: 不是 is the only negation of 是. Why is it so reliable? Because 没 negates the completion of an action or the existence/possession of something (see 有 and 没有) — it says "it didn't happen" or "there isn't any." But 是 is not an action that can happen or fail to happen; it states an identity, a timeless equation of A with B. An identity is either true or false, never "not-yet-completed," so 没 has nothing to bite on. 不 — the negator of states, qualities, and general truths — is the natural fit.

我不是记者，我是老师。

wǒ bú shì jìzhě, wǒ shì lǎoshī

I'm not a reporter, I'm a teacher. (neutral) — 不是, the only negation of 是.

这不是我的，是我室友的。

zhè bú shì wǒ de, shì wǒ shìyǒu de

This isn't mine, it's my roommate's. (informal) — 不是 links to a possessive noun phrase.

A crucial corollary: past-tense "wasn't" is still 不是. Because 没 is barred, the sentence carries its pastness through a time word (以前, 那时候, 去年), not through the negator. "He wasn't a teacher back then" is 他以前不是老师 — 不是 stays put, and 以前 supplies the past.

他以前不是老师，是工程师。

tā yǐqián bú shì lǎoshī, shì gōngchéngshī

He wasn't a teacher before — he was an engineer. (neutral) — pastness comes from 以前; the negator stays 不是.

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The tone of 不 changes here. Before the fourth-tone 是, 不 (normally bù) shifts to rising bú — 不是 = bú shì. This is the regular 不-sandhi rule, covered on 不 tone change. Say it as one falling-then-rising unit.

Once the plain 不是 is secure, two frames build on it. Here's the map before we take them one at a time:

Frame	Meaning	Example
不是 A	is not A	我不是学生 (I'm not a student)
不是 A，而是 B	not A, but rather B (correction)	不是老师，而是学生 (not a teacher, but a student)
不是 A，就是 B	if not A, then B (one of two)	不是加班，就是睡觉 (either working late or sleeping)

不是…而是…: "not X, but rather Y"

The most elegant thing 不是 does is set up a correction. The frame 不是 A，而是 B rejects A and substitutes B — "it's not A, but rather B." 而 (ér) is a light literary connector meaning "but rather / instead," and it pairs naturally with the 不是 that opened the clause. This is the go-to pattern for correcting a misunderstanding cleanly. Its own page is 不是…而是.

他不是老师，而是学生。

tā bú shì lǎoshī, ér shì xuésheng

He isn't a teacher, but a student. (neutral) — the classic 不是…而是 correction.

我要的不是这个颜色，而是那个。

wǒ yào de bú shì zhège yánsè, ér shì nàge

The colour I want isn't this one, it's that one. (informal) — 而是 substitutes the right option.

重要的不是输赢，而是过程。

zhòngyào de bú shì shū yíng, ér shì guòchéng

What matters isn't winning or losing, but the process. (neutral, slightly formal) — 不是…而是 for a considered contrast.

In speech you can often drop 而 and just say 不是 A，是 B (as in the opening examples), which is a touch more colloquial; 而是 is a little more formal and written.

不是…就是…: "if not X, then Y"

Swap 而是 for 就是 and the meaning flips from correction to alternation: 不是 A，就是 B means "it's either A or B" — one of these two, with nothing else in the picture. It often carries a mild note of "that's all it ever is."

他周末不是加班，就是睡觉。

tā zhōumò bú shì jiābān, jiù shì shuìjiào

On weekends he's either working overtime or sleeping. (informal) — 不是…就是 = one of these two.

这几天不是刮风，就是下雨。

zhè jǐ tiān bú shì guā fēng, jiù shì xià yǔ

These past few days it's been either windy or rainy. (informal) — the 'nothing but these two' flavour.

Corrective, stressed 是

The mirror image of 不是 is a stressed 是 that insists on a truth against a doubt or a wrong assumption. Where 不是 says "no, not that," a heavily stressed 是 says "yes, it really IS that." The two often appear together — 是 A，不是 B — to correct one label with another.

他是我哥，不是我弟。

tā shì wǒ gē, bú shì wǒ dì

He's my older brother, not my younger one. (informal) — stressed 是 corrects the wrong assumption.

我没开玩笑，这件事是真的。

wǒ méi kāi wánxiào, zhè jiàn shì shì zhēn de

I'm not joking — this really is true. (informal) — 是 affirms against disbelief; note 没 negates the action 开玩笑, while 是 stands unnegated.

That last example is worth a second look: 没 attaches to the action 开玩笑 ("didn't joke"), exactly as expected, while the identity clause keeps 是. Two negators, each doing its own job — a good snapshot of why 是 never borrows 没.

Common Mistakes

1. Negating 是 with 没. The cardinal error. 是 takes only 不.

❌ 他没是我的老师。

Incorrect — 是 is never negated with 没. Say 他不是我的老师.

✅ 他不是我的老师。

tā bú shì wǒ de lǎoshī

He isn't my teacher. — 不是, with 不 rising to bú before 是.

2. Using 没是 for the past. "Wasn't" is still 不是 — the past comes from a time word.

❌ 我去年没是学生。

Incorrect — no 没是. Say 我去年不是学生, with 去年 carrying the past.

✅ 我去年不是学生，是上班族。

wǒ qùnián bú shì xuésheng, shì shàngbānzú

Last year I wasn't a student, I was working. — 去年 supplies the past; 不是 stays.

3. Using 但是 instead of 而是 in the correction. 而是 substitutes (not A but B); 但是 merely contrasts (A, but…). They aren't interchangeable.

❌ 他不是老师，但是学生。

Wrong connector — 但是 means 'but/however', not 'but rather'. The substitution frame is 而是.

✅ 他不是老师，而是学生。

tā bú shì lǎoshī, ér shì xuésheng

He isn't a teacher, but a student. — 而是 substitutes the correct term.

4. Answering "no" with a bare 不. English "No." tempts learners to reply with a lone 不, but the natural negative answer to a 是 question is the full 不是.

❌ ——他是你哥吗？——不。

Unnatural — a bare 不 doesn't stand as an answer here. Say 不是.

✅ ——他是你哥吗？——不是，他是我表哥。

tā shì nǐ gē ma? — bú shì, tā shì wǒ biǎogē

— Is he your older brother? — No, he's my cousin. — 不是 answers 'no', with 不 rising to bú before 是.

Key Takeaways

是 is negated only with 不 → 不是 (bú shì). 没是 does not exist, because 是 states a timeless identity that 没 (completion / existence) cannot touch.
Past "wasn't" is still 不是; a time word like 以前 / 去年 carries the pastness.
不是…而是… = "not X, but rather Y" (a correction). 不是…就是… = "if not X, then Y" (one of two).
A stressed 是 insists on a truth ("it really IS"), often paired as 是 A，不是 B.
Remember the sandhi: 不 → bú before 是.

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