What is the difference between 不得 and 不能?

Both can mean 'must not,' but 不得 is formal and used in written rules, laws, and signs. 不能 is more general and can mean either 'cannot' (inability) or 'must not' (prohibition) in everyday speech.

When should I use 不得 instead of 不能?

Use 不得 in formal, official contexts: signs, regulations, contracts, and legal notices. If you want a strong, authoritative prohibition, choose 不得. For casual or spoken prohibitions, use 不能.

Can 不能 be used in formal writing?

Yes, but it depends on the context. In formal bans (e.g., laws), 不得 is preferred. In academic or professional writing that describes inability, 不能 is fine. For prohibitions, if the tone is less authoritative, 不能 can appear, but 不得 is more typical.

Is there an affirmative form of 不得?

No. 不得 only appears in the negative. To express 'must' or 'shall' in formal contexts, use 必须 (bìxū) or 应当 (yīngdāng).

Easily confusedHSK 6

不得 vs 不能 (bùdé vs bùnéng): formal prohibition versus inability

Both 不得 and 不能 can express prohibition, but they differ in register and context. 不得 is formal and legalistic, meaning 'must not' or 'may not,' used in rules, signs, and regulations. 不能 is a general term for 'cannot' that covers both inability and everyday prohibition (e.g., 'you must not'). Choosing the wrong one sounds unnatural or inappropriate for the setting.

The core distinction is register and scope. 不得 is a formal, often legal or administrative term meaning 'must not' or 'may not'; it appears in written rules, notices, and official documents. 不能 is much broader: it can mean 'cannot' (inability, lack of skill) or 'must not' (prohibition) in everyday, informal to moderately formal contexts. In prohibitive contexts, using 不能 where 不得 is expected sounds too casual, while using 不得 in daily speech sounds stiff or bureaucratic.

When to use each

不得bù dé

must not / may not

Use 不得 in formal, legal, or administrative contexts to state a prohibition. It is common on signs, in regulations, contracts, and official announcements. It implies a rule that is not to be violated.

不得 does not express inability; it only expresses prohibition. It is inherently negative (no affirmative counterpart meaning 'must' – use 必须 or 要 instead).

不能bù néng

cannot / may not

Use 不能 for both inability (e.g., 'I can't swim') and everyday prohibition (e.g., 'You can't park here'). It is appropriate in spoken and informal written language. Context determines whether it means 'not able to' or 'not allowed to.'

In prohibitive uses, 不能 is less forceful and less formal than 不得. For strong, official bans, 不得 is preferred.

At a glance

	不得	不能
Register	Formal, legal, administrative	Informal to formal (broad range)
Primary meaning	Prohibition ('must not')	Inability or prohibition ('cannot')
Common contexts	Signs, regulations, laws, contracts	Daily conversation, ability statements, casual prohibitions
Affirmative form	No direct affirmative (use 必须/要)	能 (able to / may)
Negation structure	Already negative (不 + 得 = must not)	Negative of 能 (cannot / may not)

Examples

不得
公共场所不得吸烟。
Gōng gòng chǎng suǒ bù dé xī yān.
No smoking in public places.
Formal sign; 不能 would be too casual here.
不能
他不能来，因为他生病了。
Tā bù néng lái, yīn wèi tā shēng bìng le.
He can't come because he is sick.
Inability, not prohibition.
不得
校园内不得停放车辆。
Xiào yuán nèi bù dé tíng fàng chē liàng.
No parking on campus.
Typical regulation; 不能 would be less authoritative.
不能
你不能这样做，这是违法的。
Nǐ bù néng zhè yàng zuò, zhè shì wéi fǎ de.
You cannot do this; it's illegal.
Everyday prohibition; 不得 sounds too formal for spoken warning.
不得
不得私自离开岗位。
Bù dé sī zì lí kāi gǎng wèi.
Do not leave your post without authorization.
Official workplace rule.
不能
我不吃辣，所以不能吃这个菜。
Wǒ bù chī là, suǒ yǐ bù néng chī zhè ge cài.
I don't eat spicy food, so I can't eat this dish.
Inability due to personal preference.

Common mistakes

Using 不能 on a formal sign (e.g., '不能吸烟' instead of '不得吸烟') — sounds too informal for regulations.
Using 不得 in casual speech (e.g., '你不得去' instead of '你不能去') — sounds overly stiff and unnatural.
Using 不得 to express inability (e.g., '我不得游泳' for 'I can't swim') — 不得 does not mean lack of ability.
Confusing 不能 with 不会 (bùhuì, 'don't know how to') in ability contexts — 不能 indicates external/inability, not learned skill.

FAQ

What is the difference between 不得 and 不能?: Both can mean 'must not,' but 不得 is formal and used in written rules, laws, and signs. 不能 is more general and can mean either 'cannot' (inability) or 'must not' (prohibition) in everyday speech.
When should I use 不得 instead of 不能?: Use 不得 in formal, official contexts: signs, regulations, contracts, and legal notices. If you want a strong, authoritative prohibition, choose 不得. For casual or spoken prohibitions, use 不能.
Can 不能 be used in formal writing?: Yes, but it depends on the context. In formal bans (e.g., laws), 不得 is preferred. In academic or professional writing that describes inability, 不能 is fine. For prohibitions, if the tone is less authoritative, 不能 can appear, but 不得 is more typical.
Is there an affirmative form of 不得?: No. 不得 only appears in the negative. To express 'must' or 'shall' in formal contexts, use 必须 (bìxū) or 应当 (yīngdāng).