When do I use 不用 vs 不得 to negate 'must'?

Only 不用 (or 不必) is used to negate 得 (děi 'must'). 不得 cannot be used this way because it means 'must not', not 'don’t have to'. Use 不用 when you want to say something is not necessary.

Is 不得 ever used in daily speech?

Rarely. 不得 is mostly found in formal rules, signs, or literary language (e.g., 不得入内 'No entry'). In everyday conversation, 'must not' is usually expressed with 不能 (bùnéng) or 不可以 (bùkěyǐ).

Can I use 不必 instead of 不用?

Yes, 不必 (bùbì) is a synonym for 不用 in the sense of 'need not'. 不必 is slightly more formal, but both are correct and commonly used. The confusion with 不得 remains the same: never use 不得 for 'need not'.

Why do some learners say 'bùděi'?

Because they incorrectly treat 得 (děi 'must') as a regular modal that can be negated with 不, producing *bùděi. However, 不得 is a standard word pronounced bùdé (fourth tone + second tone), with a completely different meaning. The form *bùděi does not exist in standard Mandarin.

Easily confusedHSK 5

不用 vs 不得 (bùyòng vs bùdé): Negating 得 (děi 'must')

得 (děi, 'must') has no direct negative form. Learners often mistakenly use 不得 (bùdé) to mean 'don't have to', but 不得 actually means 'must not' in formal/literary contexts. The correct way to say 'don't have to' is 不用 (bùyòng) or 不必 (bùbì).

得 (děi) is a modal verb meaning 'must' or 'have to'. Unlike most modals, it does not have a direct negation with 不. Learners often incorrectly insert 不 before 得 to form 不得 (bùdé), but that form already exists with a different meaning—'must not' or 'not allowed', used only in formal, fixed expressions (e.g., 不得入内 'no entry'). To express the logical negation of 得 ('don't have to'), the correct forms are 不用 (bùyòng) or 不必 (bùbì), both meaning 'need not'. Understanding this distinction prevents a common and noticeable error.

Cuándo usar cada uno

不用bú yòng

need not; don't have to

Use 不用 when you want to say that something is not necessary or required, i.e., the opposite of 得 (must). It is the standard negation in both spoken and written Chinese. It can also mean 'don't need to use', but in the context of modal negation it always expresses lack of necessity.

In some dialects or colloquial speech, 不用 can be shortened to 甭 (béng), but 不用 is always correct. 不用 does not carry any prohibition meaning.

不得bù dé

must not; not allowed (formal/literary)

不得 is used in formal or written language to express prohibition, often in signs, rules, or legal texts (e.g., 不得喧哗 'no loud talking'). It NEVER means 'don't have to' and is NOT the negation of the modal 得 (děi).

Many learners mispronounce 不得 as 'bùděi' (with the same tone as 得 děi) and use it to mean 'don't have to', but this is a non-standard, erroneous form. The correct reading is bùdé, with a second tone on 得. In modern daily speech, 不得 is rare; 不能 (bùnéng) or 不可以 (bùkěyǐ) are more common for 'must not'.

De un vistazo

	不用	不得
Meaning	Need not; don't have to	Must not; not allowed
Negation of 得 (děi 'must')	Used as the direct negation (correct)	Not the negation; incorrect in this role
Register	Neutral; used in both speech and writing	Formal/literary; rarely used in conversation
Typical usage	Expressing the absence of necessity ('you don’t need to…')	Expressing prohibition ('you must not…')

Ejemplos

不用
你不用太担心。
Nǐ bú yòng tài dān xīn.
You don’t have to worry too much.
Correct negation of 得 (děi) — here replacing 得 would give '你不用太担心' as the natural opposite of '你得担心'
不用
明天是周末，我不用上学。
Míng tiān shì zhōu mò, wǒ bú yòng shàng xué.
Tomorrow is the weekend, I don’t have to go to school.
Expressing lack of necessity — 不得 would be wrong here.
不得
此处不得吸烟。
Cǐ chù bù dé xī yān.
Smoking is not allowed here.
Formal prohibition sign — 不用 would not convey prohibition.
不得
考试时不得作弊。
Kǎo shì shí bù dé zuò bì.
Cheating during exams is not allowed.
Formal rule — using 不用 would mean 'don’t need to cheat', which is opposite.
不得
✗你不得去，你不必去。（错误）
✗ Nǐ bù dé qù, nǐ bú bì qù. (cuò wù)
✗ You don’t have to go, you need not go. (error)
✗ Learner mistakenly writes '不得' (bùděi) meaning 'don't have to' — this form does not exist; correct is '你不用去' or '你不必去'.
不用
你不用带伞，今天不会下雨。
Nǐ bú yòng dài sǎn, jīn tiān bú huì xià yǔ.
You don’t need to bring an umbrella; it won’t rain today.
Natural everyday negation of necessity.

Errores comunes

Writing '不得' (bùdé) to mean 'don’t have to' instead of '不用' (bùyòng). E.g., '你不得去' is wrong for 'You don’t have to go'.
Pronouncing '不得' as 'bùděi' (with the same tone as 得 děi) — standard pronunciation is bùdé.
Using '不得' in spoken conversation for 'must not'; instead, use '不能' or '不可以' in most situations.
Assuming all modal verbs can be negated by adding 不 directly in front (e.g., 得→不得, 应该→不应该), but 得 is an exception.

Preguntas frecuentes

When do I use 不用 vs 不得 to negate 'must'?: Only 不用 (or 不必) is used to negate 得 (děi 'must'). 不得 cannot be used this way because it means 'must not', not 'don’t have to'. Use 不用 when you want to say something is not necessary.
Is 不得 ever used in daily speech?: Rarely. 不得 is mostly found in formal rules, signs, or literary language (e.g., 不得入内 'No entry'). In everyday conversation, 'must not' is usually expressed with 不能 (bùnéng) or 不可以 (bùkěyǐ).
Can I use 不必 instead of 不用?: Yes, 不必 (bùbì) is a synonym for 不用 in the sense of 'need not'. 不必 is slightly more formal, but both are correct and commonly used. The confusion with 不得 remains the same: never use 不得 for 'need not'.
Why do some learners say 'bùděi'?: Because they incorrectly treat 得 (děi 'must') as a regular modal that can be negated with 不, producing *bùděi. However, 不得 is a standard word pronounced bùdé (fourth tone + second tone), with a completely different meaning. The form *bùděi does not exist in standard Mandarin.