Modal verbs — obligation / permission / ability / volition
Mandarin modals are short, frequent, and slippery — many of them overlap with English 'can / should / must / will'. Lessons 10 and 13 introduced 想 / 要 / 能 / 会 / 可以. This lesson lines up the full set and sharpens the distinctions: 应该 / 应当 (should — moral / expected), 必须 (must — strong obligation), 得 děi (must, colloquial), 可以 (can / may — permission or possibility), 能 (be able to — circumstantial / capacity), 会 (know how to — learned skill / will — likelihood), 要 (want / need to / will), 想 (want / would like — soft). All sit BEFORE the main verb. The differences are subtle but native speakers track them precisely. Get this map straight and you'll stop saying 可以游泳 when you mean 会游泳.
应该 / 应当 — should (moral or expected)
应该 yīnggāi = 'ought to / should'. It expresses a moral or normative expectation: this is the right thing, the proper thing, what people are supposed to do. 你应该听妈妈的话。 = You should listen to mom. 我们应该早点儿到。 = We should arrive a bit early. 他应该已经到了。 = He should have arrived already (an inference / expectation). The negative is 不应该 = shouldn't: 你不应该这么说。 = You shouldn't talk like this. 应当 yīngdāng is a slightly more formal / written synonym — common in essays, less so in chat. In conversation, 应该 carries everything. 应该 is softer than 必须 (must) — it advises, it doesn't command. Compare: 你应该多休息 (you should rest more — friendly advice) vs. 你必须休息 (you must rest — doctor's orders).
必须 — must (strong external obligation)
必须 bìxū is the strongest 'must' — a hard requirement, often coming from outside (rules, schedules, authority). 学生必须做作业。 = Students must do their homework. 你必须现在去。 = You must go now. 必须 doesn't accept softening — there's no real 'maybe should'. The negation is unusual: 不必须 is rare; speakers say 不必 búbì = 'don't have to / no need to': 你不必去。 = You don't have to go. Or 不用 (lesson 35-coined / common): 你不用去 = same. So the 必须 / 不必 / 不用 split is irregular — affirmative uses 必须, negative usually uses 不必 or 不用. 必须 is a touch formal — it appears on signs, in announcements, in workplace rules — and is fully natural in deliberate speech.
得 děi — must / have to (colloquial)
得 děi (note the special pronunciation — different from dé 'get' or de the structural particle from lesson 28) is the colloquial 'have to / must'. It's the everyday spoken counterpart to 必须. 我得走了。 = I have to go (now). 你得吃药。 = You have to take your medicine. 得 is informal, conversational, and very frequent in Beijing-flavored speech. The negation is again 不必 / 不用 (the 不得 form is reserved for other meanings). 必须 is slightly more formal, 得 is more casual — both translate to English 'have to / must'. Mixing them in the same sentence is fine, but native speakers rarely use 必须 in chat with friends.
可以 — can / may (permission or possibility)
可以 kěyǐ is the broad 'can / may'. Two main uses: (1) PERMISSION — 'allowed to': 你可以走了。 = You may go now. 这里可以抽烟吗? = May (one) smoke here? (2) POSSIBILITY — 'it is possible / it works': 这本书可以借两个星期。 = This book can be borrowed for two weeks. 可以 does NOT mean 'know how to' — that's 会. Saying 我可以游泳 sounds like 'I'm permitted to swim' or 'it's possible for me to swim (right now)', NOT 'I know how to swim'. For skill, use 我会游泳. The negation 不可以 = 'not allowed': 不可以进 = no entry. Note: questioning permission, learners often produce 可以...吗? which is correct: 我可以问一个问题吗? = May I ask a question?
能 — be able to (circumstantial capacity)
能 néng = 'can' in the sense of being physically / circumstantially able. The conditions are right; the action is doable. 我今天能去。 = I can go today (I'm free / it's possible for me). 你能听见吗? = Can you hear (me)? 他能跑很快。 = He can run very fast (capacity). 能 also overlaps with permission in some uses: 这里能抽烟吗? also works (≈ 可以). The crispest distinction is: 能 = able to (right now, given circumstances), 可以 = allowed to / it works, 会 = know how to (learned skill). 我会开车 = I know how to drive (passed the test). 我能开车 = I can drive (right now — I'm sober, the car works). 我可以开车 = I'm allowed to drive (have permission). In real speech these blur, but the cleanest test: did you LEARN it (会), is it physically possible right now (能), or are you allowed (可以)?
会 — know how to / will (likelihood)
会 huì is double-duty. (1) LEARNED SKILL — 'know how to': 我会说中文。 = I know how to speak Chinese (I learned). 他不会开车。 = He doesn't know how to drive. The negation 不会 = doesn't know how. This is the most common student use. (2) LIKELIHOOD / PREDICTION — 'will, would': 明天会下雨。 = It will rain tomorrow. 他会来吗? = Will he come? Here 会 is about probability, not skill. Distinguish from 要 (intention / planned will): 我要去 = I'm going (planned), 我会去 = I'll go / I would go (likely / conditional). For 'I can swim' (skill) only 会 is right: 我会游泳. For 'I can swim today' (circumstance) only 能 is right: 我今天能游泳.
要 — want / need to / will (intention)
要 yào is broad — three main flavors. (1) WANT (concrete object): 我要一杯咖啡。 = I want a cup of coffee. (2) GOING TO / WILL (intention or scheduled future): 我明天要去北京。 = I'm going to Beijing tomorrow. 快要下雨了。 = It's about to rain. (3) NEED TO / HAVE TO (mild obligation): 你要小心。 = You need to be careful. 要 as future is stronger than 会 (likelihood) — 要 signals a definite plan. Negation forks: 不要 = don't (prohibitive command, like 'don't'): 不要走! = Don't go! For 'don't want', use 不想 (soft) or context. 不要 is strong — between strangers it can sound rude — soft alternatives: 请不要 (please don't), 别 (don't, lesson 26).
想 — want / would like (soft volition)
想 xiǎng is the soft 'want'. From lesson 10: 我想喝咖啡。 = I'd like a coffee. Compared with 要 (definite, even pushy), 想 is wishful, polite, hypothetical. 我想去北京。 = I'd like to go to Beijing (someday). 我要去北京。 = I'm going to Beijing (planned). The negation 不想 = don't want to (preference): 我不想去。 = I don't want to go. 想 also has a non-modal meaning — 'to think / miss' — when followed by an object (not a verb): 我想你。 = I miss you. 我想想。 = Let me think. Modal 想 + V; non-modal 想 + N or by itself.
Pairings and combinations
Modals can stack and combine, but only in fixed orders. 想 + V (want to V), 能 + V (be able to V), but 想能 + V is wrong; you'd say 我希望能去 (I hope I can go) for that idea. Common natural combinations: 应该可以 = should be able to (probability + permission): 明天应该可以去。 = Tomorrow should be possible. 得 + 会 is rare; instead: 必须学会 = must learn (and master). 要 + 会 doesn't combine; use 要学会 = need to learn. Quick reference table by English intent: 'want to' soft = 想; 'want to' firm = 要; 'know how to' = 会; 'be able to (now)' = 能; 'be allowed to' = 可以; 'should' = 应该; 'must (formal)' = 必须; 'have to (chat)' = 得. Memorizing the row makes Mandarin modal use click.
