必须 (bìxū) is Mandarin's strong, categorical "must." Where 应该 (yīnggāi) advises and 得 (děi) says "gotta" in a friendly way, 必须 lays down a requirement — often external, rule-based, or emphatic. It's the word on signs, in regulations, in instructions, and in advice you really mean: 乘客必须系安全带 ("passengers must fasten their seatbelts"). Think of it as English "must / have to" at its most obligatory. It reads as firmer and more formal than 得, which is why 得 dominates casual chat and 必须 dominates rules and written notices. Two things to nail down: its negation is not 不必须, and it has a soundalike cousin, 必需, that is a completely different word.
必须 for strong, categorical obligation
必须 sits before a verb and states that the action is required — no wiggle room. It suits rules, deadlines, conditions, and forceful advice.
你必须先完成今天的作业，才能玩游戏。
nǐ bìxū xiān wánchéng jīntiān de zuòyè, cái néng wán yóuxì
You must finish today's homework first, then you can play games. (neutral) — 必须 = a firm condition.
乘客必须系好安全带。
chéngkè bìxū jì hǎo ānquándài
Passengers must fasten their seatbelts. (formal) — 必须 on a rule/regulation.
报名必须在本周五之前完成。
bàomíng bìxū zài běn zhōuwǔ zhīqián wánchéng
Registration must be completed by this Friday. (formal) — 必须 with a hard deadline.
想在这行做出成绩，就必须肯下功夫。
xiǎng zài zhè háng zuòchū chéngjì, jiù bìxū kěn xià gōngfu
To make a mark in this field, you simply must be willing to put in the work. (neutral) — emphatic 必须.
Because 必须 scopes the whole requirement, it can sit right after the subject and take the entire clause under its wing — 所有人必须… ("everyone must…"), 你必须… ("you must…"). It commonly heads notices where the subject is a whole category of people.
所有员工上班时必须佩戴工牌。
suǒyǒu yuángōng shàngbān shí bìxū pèidài gōngpái
All staff must wear their ID badges at work. (formal) — 必须 governing a whole clause.
必须 vs 得: same idea, different register
必须 and 得 (děi) both mean "must / have to," and often either works. The split is register and force. 必须 is formal, categorical, at home in writing and rules; 得 is colloquial, warm, the spoken default. Compare the same duty in a notice versus in chat.
| Modal | Strength | Register | Typical home |
|---|---|---|---|
| 应该 (yīnggāi) | advice — "should" | neutral | suggestions, opinions |
| 得 (děi) | necessity — "gotta" | colloquial, spoken | everyday conversation |
| 必须 (bìxū) | strong, categorical — "must" | formal, emphatic | rules, notices, firm advice |
通知：所有访客必须登记后方可进入。
tōngzhī: suǒyǒu fǎngkè bìxū dēngjì hòu fāng kě jìnrù
Notice: all visitors must register before entering. (formal) — 必须 on an official notice.
行了，我先去登个记，一会儿见。
xíng le, wǒ xiān qù dēng ge jì, yíhuìr jiàn
Okay, I'll go sign in first — see you in a bit. (informal) — the same duty, told casually with 得-style ease.
The finer three-way comparison of 应该 / 得 / 必须 lives on the obligation modals contrast page; for the friendly spoken counterpart see 得 (děi).
Negation: 不必 / 不用, never 不必须
This is the classic error. The opposite of "must" is "need not," and its word is 不必 (búbì) or 不用 (búyòng) — not 不必须. You don't negate 必须 by sticking 不 in front of it.
这些细节你不必都记住，用到时查一下就行。
zhèxiē xìjié nǐ búbì dōu jì zhù, yòng dào shí chá yíxià jiù xíng
You don't have to memorize all these details — just look them up when you need them. (neutral) — 不必 = need not.
无须担心，一切都在正常运转。
wúxū dānxīn, yíqiè dōu zài zhèngcháng yùnzhuǎn
No need to worry — everything's running normally. (formal) — 无须, the formal 'need not'.
And beware a second trap: "must not" is not the negation of 必须 either. "Need not" (不必) and "must not" (不能 / 不得 / 别) are two different negatives. 你不必去 = "you don't have to go"; 你不能去 = "you're not allowed to go."
考试时不能看手机，否则算作弊。
kǎoshì shí bù néng kàn shǒujī, fǒuzé suàn zuòbì
You must not look at your phone during the exam, or it counts as cheating. (formal) — prohibition is 不能, not a negated 必须.
必须 vs 必需: two words, one sound
必须 has a homophone, 必需 (also bìxū), and they are not interchangeable. 必须 modifies a verb — "must do." 必需 modifies a noun — "essential, indispensable." The tell is what follows: an action, or a thing.
水和空气是生活的必需品。
shuǐ hé kōngqì shì shēnghuó de bìxūpǐn
Water and air are life's necessities. (neutral) — 必需品 'necessities': 必需 + noun.
签证是入境必需的文件，你必须提前办好。
qiānzhèng shì rùjìng bìxū de wénjiàn, nǐ bìxū tíqián bàn hǎo
A visa is a required document for entry, and you must arrange it in advance. (formal) — 必需 (essential, + noun) and 必须 (must, + verb) in one sentence.
Common Mistakes
1. Negating with 不必须. The negation of 必须 is 不必 or 不用 — 不必须 is not a word.
❌ 这件事你不必须亲自去。
Wrong — 'you needn't go in person' is 不必 / 不用, not 不必须.
✅ 这件事你不必亲自去。
zhè jiàn shì nǐ búbì qīnzì qù
You don't have to go in person for this. — 不必.
2. Using a negated "must" for "must not." "Need not" (不必) and "must not" (不能 / 不得) are different. Don't say 必须不 for a prohibition.
❌ 你必须不告诉别人。
Awkward — for 'you mustn't tell anyone' use a ban: 你不能告诉别人.
✅ 这件事你不能告诉别人。
zhè jiàn shì nǐ bù néng gàosu biérén
You must not tell anyone about this. — 不能 for a prohibition.
3. Mixing up 必须 and 必需. Verb → 必须; noun → 必需. Getting them swapped is a spelling-level error natives notice at once.
❌ 这些是生活必须品。
Wrong character — 'necessities' (a noun) takes 必需: 生活必需品.
✅ 这些是生活必需品。
zhèxiē shì shēnghuó bìxūpǐn
These are daily necessities. — 必需 + noun 品.
4. Using formal 必须 in casual speech where 得 is natural. 必须 in a relaxed chat can sound stiff or bossy; among friends, 得 is warmer.
❌ 时间不早了，我必须回家了。
Over-formal for chatting with friends — 得 sounds more natural: 我得回家了.
✅ 时间不早了，我得回家了。
shíjiān bù zǎo le, wǒ děi huí jiā le
It's getting late — I gotta head home. — colloquial 得.
Key Takeaways
- 必须 (bìxū) is the strong, categorical "must" — rules, deadlines, notices, and firm advice: 乘客必须系安全带.
- It's firmer and more formal than 得 (děi, the spoken "gotta") and stronger than 应该 (advice).
- Negation is 不必 / 不用 ("need not") — never 不必须. And "must not" is a different negative: 不能 / 不得 / 别.
- 必须 (must, + verb) ≠ 必需 (essential, + noun) — same sound, different jobs: 你必须做 vs 生活必需品.
- Reach for the colloquial 得 in casual speech; save 必须 for rules and emphasis.
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- 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.
- 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.
- Choosing 应该 / 得 / 必须 / 要 / 需要HSK 3 — The 'should / must / need to' cluster on one strength scale — 应该 (advisable) < 要 / 需要 (need to) < 得 děi (gotta) < 必须 (categorical must) — and the negation minefield: 不应该 = shouldn't, 不用 / 不必 = needn't, 不能 / 别 = must not. You almost never negate 得 or 必须 directly.
- 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.