English hides an entire strength scale inside two words, should and have to — and then, in the negative, it does something treacherous: "you don't have to" and "you must not" are near-opposites, yet both are built from "have to." Chinese refuses to bundle these. It spreads should → gotta → must across four distinct words, and it uses completely different words for "needn't" and "must not." Get the positive scale and the negation map straight and this whole cluster — one of the genuine trouble spots for English speakers — clicks into place.
The strength scale
Ranked from softest advice to hardest requirement:
| Modal | Pinyin | Force | Feel |
|---|---|---|---|
| 应该 | yīnggāi | should, ought to | advice — the right/sensible thing |
| 要 / 需要 | yào / xūyào | need to, have to | practical necessity, everyday |
| 得 | děi | gotta, have to | colloquial, spoken; felt pressure |
| 必须 | bìxū | must, categorically | rule, formal, no exceptions |
- 应该 (yīnggāi) — the mildest. It says an action is advisable or morally right, and it leaves room to disagree.
- 要 / 需要 — plain practical necessity: this needs doing. 要 is the everyday spoken workhorse; 需要 is the more explicit "need to."
- 得 (děi) — colloquial "gotta." It carries a felt, often reluctant pressure and is almost purely spoken — you rarely write it.
- 必须 (bìxū) — the categorical "must" of rules, regulations, and formal writing. No wiggle room.
One situation, five strengths
Watch the same piece of advice climb the scale as you swap the modal:
| Sentence | Force |
|---|---|
| 你应该早点睡 (nǐ yīnggāi zǎo diǎn shuì) | You should sleep earlier — friendly advice |
| 你需要早点睡 (nǐ xūyào zǎo diǎn shuì) | You need to sleep earlier — it's necessary |
| 你得早点睡 (nǐ děi zǎo diǎn shuì) | You've gotta sleep earlier — spoken, pressing |
| 你必须早点睡 (nǐ bìxū zǎo diǎn shuì) | You must sleep earlier — an order, no argument |
你脸色这么差，应该早点睡，别老熬夜。
nǐ liǎnsè zhème chà, yīnggāi zǎo diǎn shuì, bié lǎo áoyè
You look so worn out — you should sleep earlier and stop staying up. (informal) — 应该 as gentle advice.
感冒了？那你得多喝水，好好休息。
gǎnmào le? nà nǐ děi duō hē shuǐ, hǎohǎo xiūxi
Caught a cold? Then you've gotta drink lots of water and rest up. (informal) — colloquial 得.
上飞机前必须出示护照和登机牌。
shàng fēijī qián bìxū chūshì hùzhào hé dēngjīpái
You must show your passport and boarding pass before boarding. (formal) — 必须 for a hard rule.
时间不早了，我得走了。
shíjiān bù zǎo le, wǒ děi zǒu le
It's getting late — I've gotta go. (informal) — 得 with a felt, spoken pressure.
The negation minefield
Here is where English trips you. "Have to" negates two opposite ways — don't have to (the requirement is lifted) vs. must not (the action is forbidden) — and Chinese uses entirely separate words for each. Crucially, you do not form them by sticking 不 on 得 or 必须. The forms 不得(děi) and 不必须 simply don't exist.
| English negative | Chinese | Meaning |
|---|---|---|
| shouldn't | 不应该 / 不该 (bù yīnggāi / bù gāi) | mild negative advice — 应该 does negate directly |
| don't have to / needn't | 不用 / 不必 / 不需要 (bú yòng / bú bì / bù xūyào) | the requirement is lifted |
| must not | 不能 / 不可以 / 别 / 不许 (bù néng / bù kěyǐ / bié / bù xǔ) | the action is forbidden |
你不用道歉，这真的不是你的错。
nǐ bú yòng dàoqiàn, zhè zhēn de bú shì nǐ de cuò
You don't have to apologize — it really isn't your fault. (informal) — 不用 lifts the obligation.
考试的时候不能用手机。
kǎoshì de shíhou bù néng yòng shǒujī
You must not use your phone during the exam. (neutral) — 不能 forbids; note it's a different word from 不用.
你不应该对妈妈那样说话。
nǐ bù yīnggāi duì māma nàyàng shuōhuà
You shouldn't talk to Mom like that. (informal) — 应该 negates cleanly to 不应该.
不必这么客气，都是自己人。
bú bì zhème kèqi, dōu shì zìjǐ rén
No need to be so polite — we're all family here. (neutral, slightly formal) — 不必, the more written twin of 不用.
The single trap to burn in: 不能来 forbids coming; 不用来 lifts the need to come. Same English scaffold ("don't have to" vs. "must not"), opposite meanings, different Chinese words. The full negation family is on the 不用 / 不必 (needn't) page.
Common Mistakes
1. Inventing 不必须. 必须 has no negated form. "Don't have to" is 不用 / 不必; "must not" is 不能 / 不许.
❌ 你不必须来。
Not a word — *不必须 doesn't exist. Say 你不用来 or 你不必来 for 'you don't have to come.'
✅ 你不用来，我一个人能搞定。
nǐ bú yòng lái, wǒ yí ge rén néng gǎodìng
You don't have to come — I can handle it alone. — 不用 lifts the requirement.
2. Trying to negate 得(děi) directly. There is no 不得(děi). Written 不得 is bùdé, a formal prohibition — the opposite of what you likely mean.
❌ 明天你不得来。
This reads as bùdé — 'you may NOT come,' a prohibition. For 'you don't have to come,' say 你不用来.
✅ 明天你不用来。
míngtiān nǐ bú yòng lái
You don't have to come tomorrow. — cancel the 得-obligation with 不用.
3. Using 不能 to mean "don't have to." 不能 forbids the action; it can't lift a requirement.
❌ 你累了就不能来。
Wrong if you mean 'no need to come' — 不能来 forbids coming. Say 你累了就不用来 ('if you're tired, you don't have to come').
✅ 你累了就不用来。
nǐ lèi le jiù bú yòng lái
If you're tired, you don't have to come. — 不用 lifts the need.
4. Reaching for 必须 to give friendly advice. 必须 is a categorical order; for a gentle suggestion it sounds harsh. Use 应该 or 得.
❌ 你必须多休息一下。
Too heavy for friendly advice — 必须 barks an order. For a suggestion, 你应该多休息 or 你得多休息.
✅ 你应该多休息一下。
nǐ yīnggāi duō xiūxi yíxià
You should rest a bit more. — 应该 for advisable, not commanded.
Key Takeaways
- The scale, softest to hardest: 应该 (should) < 要 / 需要 (need to) < 得 děi (gotta, spoken) < 必须 (categorical must).
- 得 is colloquial/spoken; 必须 is formal and admits no exceptions — pick by register as much as by strength.
- English "have to" negates two opposite ways; Chinese uses different words: 不用 / 不必 / 不需要 = "needn't" (obligation lifted), 不能 / 不可以 / 别 / 不许 = "must not" (forbidden).
- 不应该 ("shouldn't") is the one regular negation — 应该 negates directly.
- You cannot negate 得(děi) or 必须 with 不: 不必须 isn't a word, and 不得 is a separate formal prohibition (bùdé). Cancel a 得/必须 obligation with *不用.
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- 应该 / 该 (yīnggāi / gāi): ShouldHSK 2 — 应该 (yīnggāi) covers both English 'should's: advice (你应该多休息 'you should rest more') and inference (他应该到了 'he should have arrived by now'). The short form 该 (gāi) is colloquial, especially 该…了 'it's time to…'. Negation: 不应该.
- 得 (děi): Gotta / Have ToHSK 3 — 得 read děi is the everyday spoken 'must / have to' (我得走了 'I gotta go'). Two traps: it's the same character as structural 得 (de) and 得 (dé) 'obtain', and its negation is NOT 不得 — 'don't have to' is 不用 / 不必.
- 必须 (bìxū): Must (Strong Necessity)HSK 3 — 必须 (bìxū) is the strong, categorical 'must' of rules and firm advice — 你必须完成作业, 乘客必须系安全带. It's firmer and more formal than 得 (děi). Its negation is 不必 / 不用 (need not), NOT 不必须; 'must not' is 不能 / 不得 / 别.
- 不用 / 不必 (búyòng / búbì): Needn'tHSK 2 — 'No need to / don't have to' is 不用 (búyòng, colloquial) or 不必 (búbì, more formal): 你不用来 'you needn't come', 不用谢 'you're welcome', 我们不必等他 'we needn't wait for him'. It negates the NECESSITY of an action — the opposite of 必须 'must' — not the action itself, which is what makes it different from prohibiting 别.