Obligation modals (应该/得/必须)
A2 能愿动词（应该、得、必须） 26 / 56
In short
Three auxiliaries cover “should” and “must,” and they line up by force: 应该 (yīnggāi, should / ought to) is advice, 得 (děi, gotta / have to) is everyday spoken necessity, and 必须 (bìxū, must) is a hard rule. All three sit right before the main verb. The catch is the negative: only 应该 flips to 不应该; 得 and 必须 have no 不- form - “needn’t” becomes 不用 (búyòng) or 不必 (búbì).
The rule
Like every modal (see Modal verbs (会/能/可以/想/要)), these are auxiliaries in a fixed slot:
Subject + (adverb) + Modal + Verb (+ Object)
So “you should rest more” is 你应该多休息 (nǐ yīnggāi duō xiūxi): subject 你, modal 应该, then the verb phrase. Any adverb goes in front of the modal. Now the three words, from softest to strongest:
应该 (yīnggāi) - should, ought to. Advice, moral duty, or what you reasonably expect. It negates cleanly to 不应该 (bù yīnggāi, shouldn’t). In speech it often shortens to just 该 (gāi): 你该走了 (nǐ gāi zǒu le, it’s time you left). 应该 has a second job - likelihood: 他应该已经到了 (tā yīnggāi yǐjīng dào le, he should have arrived by now), an inference, not a duty.
得 (děi) - have to, gotta. Colloquial, spoken necessity - what circumstances force on you. 我得走了 (wǒ děi zǒu le, I’ve got to go). Two things to nail. First the sound: this modal is děi, 3rd tone. The character 得 also spells dé (the verb “to obtain”) and the neutral-tone particle de that follows a verb to mark manner - that particle belongs to Manner and state complement (V 得…) and is a different structure entirely. The obligation one is always děi and always before the verb. Second: 得 (děi) has no negative of its own - you never say 不得 for “don’t have to”; use 不用 / 不必 instead.
必须 (bìxū) - must. Strong, often written or official: rules, requirements, conditions. 你必须按时完成作业 (nǐ bìxū ànshí wánchéng zuòyè, you must finish the homework on time). It, too, takes no 不 directly (不必须 is wrong): “needn’t” is 不必 (búbì) or 不用, and “must not / it’s forbidden” is 不能 (bù néng) or 不许 (bùxǔ).
Negation is not symmetric - the one point to memorize. 应该 → 不应该 works. 得 and 必须 do not - reach for a different word depending on the meaning. Note the tone sandhi: 不 becomes bú before a 4th tone, so it is búbì (不必) and búyòng (不用). See the table below. (Written 不得 bùdé does exist, but it means “may not / is forbidden” - a formal prohibition, not “needn’t.”)
Questions. Add 吗 (你应该去吗？), or use A-not-A on 应该: 应不应该 / 该不该. 必须 is usually questioned with 吗 or 是不是, and 得 (děi) is rarely put into a question at all - use 用不用 (yòng bu yòng) instead.
Forms & patterns
The three obligation modals side by side:
| Modal | Pinyin | Force / register | Example |
|---|---|---|---|
| 应该 (该) | yīnggāi (gāi) | should, ought to - advice, expectation | 你应该多喝水 (nǐ yīnggāi duō hē shuǐ) - you should drink more water |
| 得 | děi | gotta, have to - colloquial, spoken | 我得走了 (wǒ děi zǒu le) - I’ve got to go |
| 必须 | bìxū | must - strong, rules and formal | 你必须准时到 (nǐ bìxū zhǔnshí dào) - you must arrive on time |
How each one is negated (this is the tricky part):
| Modal | Positive | ”needn’t / don’t have to" | "shouldn’t / must not” |
|---|---|---|---|
| 应该 | 应该 (yīnggāi) | 不用 / 不必 | 不应该 (bù yīnggāi) - shouldn’t |
| 得 | 得 (děi) | 不用 (búyòng) / 不必 (búbì) | 不能 (bù néng) / 不许 (bùxǔ) |
| 必须 | 必须 (bìxū) | 不必 (búbì) / 不用 | 不能 / 不许 (bùxǔ) / 不准 (bùzhǔn) |
Examples
- 你应该早点睡。 (Nǐ yīnggāi zǎodiǎn shuì.) - You should go to bed earlier.
- 我们应该帮助他。 (Wǒmen yīnggāi bāngzhù tā.) - We ought to help him.
- 你不应该说谎。 (Nǐ bù yīnggāi shuōhuǎng.) - You shouldn’t lie.
- 时间不早了，我得走了。 (Shíjiān bù zǎo le, wǒ děi zǒu le.) - It’s getting late; I have to go.
- 明天有考试，我得复习。 (Míngtiān yǒu kǎoshì, wǒ děi fùxí.) - There’s a test tomorrow, so I’ve got to review.
- 坐飞机必须带护照。 (Zuò fēijī bìxū dài hùzhào.) - To fly you must bring your passport.
- 学生必须穿校服。 (Xuésheng bìxū chuān xiàofú.) - Students must wear a uniform.
- 你不必担心。 (Nǐ búbì dānxīn.) - You don’t need to worry.
- 他应该已经到家了。 (Tā yīnggāi yǐjīng dào jiā le.) - He should be home by now.
Common mistakes
- 我不得走。 -> 我不用走。 (Wǒ búyòng zǒu.) - 得 (děi) has no 不- form; “don’t have to / needn’t” is 不用 or 不必.
- 你不必须来。 -> 你不必来。 (Nǐ búbì lái.) - never put 不 in front of 必须; its “needn’t” partner is 不必.
- 你应该不来。 -> 你不应该来。 (Nǐ bù yīnggāi lái.) - to say “shouldn’t,” 不 goes before the modal, giving 不应该.
- 我走得快点儿。 (meaning “I must hurry off”) -> 我得快点儿走。 (Wǒ děi kuài diǎnr zǒu.) - the obligation 得 is děi and comes before the verb; V + 得 (de) after a verb marks manner, a different structure.
Tips
- Sort them by strength: 应该 = advice (“you should”), 得 / 必须 = necessity (“you have to”). 得 is what you actually say out loud; 必须 sounds official and can land like an order, so for a gentle nudge reach for 应该.
- The character 得 has three jobs - dé “to obtain” (a full verb), the neutral-tone particle de after a verb (manner), and this modal děi. Same shape, three sounds; the obligation one is always děi and sits before the verb.
- Remember the asymmetry: only 应该 flips to 不应该. For 得 and 必须, switch words - 不用 / 不必 for “needn’t,” 不能 / 不许 for “forbidden.”
- False friend to watch: 必须 (bìxū, “must,” before a verb) vs 必需 (bìxū, “essential,” an adjective, as in 必需品 bìxūpǐn “necessities”). Identical sound, different character and job.