必须
bì xūThe strongest everyday modal of obligation. When a speaker chooses 必须 over 应该 or 要, they are flagging that the matter is not up for discussion: there is no acceptable alternative, the requirement is structural rather than advisory.
必 bì (certainly; must; inevitably): the early form depicts a long handle with a binding mark, often analyzed as a shaft fastened with a slanting peg. The semantic extension reads as fastening: what is bound cannot be otherwise. By the classical period 必 had become the standard adverb of necessity, occurring in proverb-form constructions (言必信，行必果, words must be trustworthy and actions must bring results) where it locks each clause to its outcome.
须 xū (to need; to require; must; whiskers): the early graph shows a face with strands of beard, the original sense being facial hair. The phonetic borrowing into the modal sense (to require, must) is well-attested by the Han period and entirely displaces the original meaning in modern usage; the beard sense survives only in fossilized compounds (胡须 húxū, beard).
必 + 须 = a doubled emphasis, certainty fused with requirement. The compound is stronger than either component alone: 必须 says both that the action is necessary and that it is inevitable, leaving no rhetorical room for the listener to negotiate.
Subject + 必须 + Verb Phrase
必须 is a modal adverb-verb that sits before the main verb. Like 应该 and 要, it does not take 了 or 着, and it is negated only with 不 + 必须 in narrow contexts; the standard negation of obligation is the separate word 不必 (bùbì, not necessary) or 不需要 (bù xūyào, no need to).
学生必须按时交作业。
Xuésheng bìxū ànshí jiāo zuòyè.
Students must turn in their homework on time.
你必须现在就走，不然就来不及了。
Nǐ bìxū xiànzài jiù zǒu, bùrán jiù láibují le.
You have to leave right now, otherwise it will be too late.
To negate the requirement, switch to 不必 or 不用:
你不必担心，我自己能解决。
Nǐ bùbì dānxīn, wǒ zìjǐ néng jiějué.
You don't have to worry, I can handle it myself.
Mandarin has at least five everyday ways to mark obligation, and choosing between them is a register decision. From strongest to weakest:
必须 bìxū: must, structurally required, no alternative. Formal and emphatic. Used in rules, regulations, instructions, and serious advice. Often appears in written notices and policy.
得 děi (note the reading: děi, not de): have to, in a colloquial obligation sense. Same strength as 必须 in feeling but the spoken-language version. 我得走了 (I have to go) is what you actually say; 我必须走 sounds like a rule rather than a person leaving.
要 yào: should, need to, going to. Weaker, more advisory. Carries an aspect of intention as well as obligation, which softens it. 你要小心 (you need to be careful) is a warning, not a rule.
应该 yīnggāi: should, ought to. Moral or expectation-based, not structural. Allows the listener room to disagree. The distance between 你必须道歉 (you must apologize) and 你应该道歉 (you ought to apologize) is the distance between a demand and a suggestion.
需要 xūyào: to need. A neutral statement of need rather than a directive. 需要预约 (need to book in advance) is informational; it does not push the listener.
| 词 Modal | 强度 Strength | 语域 Register | 例 Typical use |
|---|---|---|---|
| 必须 bìxū | Strongest | Formal, written, emphatic | Rules, instructions, non-negotiable demands |
| 得 děi | Strong | Colloquial | Personal obligations spoken aloud |
| 要 yào | Medium | Neutral | Advice, near-future plans, warnings |
| 应该 yīnggāi | Soft | Neutral | Moral expectation, suggestion |
| 需要 xūyào | Neutral | Informational | Statement of requirement, not a directive |
Two near-homophones that even native speakers slip on. The distinction is grammatical rather than semantic.
必须 bìxū: 必 + 须. A modal adverb modifying a verb. It tells you that an action must happen. Always followed by a verb phrase.
你必须出席。 , You must attend. (modifying the verb 出席)
必需 bìxū: 必 + 需. An adjective or noun-phrase modifier meaning essential, necessary. It describes a thing rather than commanding an action. Usually followed by 的 + noun, or stands as a noun.
水是生活的必需品。 , Water is a necessity of life. (modifying 品, an item)
Quick test: if you can replace it with "must (do)," use 必须. If you can replace it with "necessary (thing)," use 必需. Get this wrong in formal writing and the reader notices immediately.