Obligation: musieć, trzeba, mieć | Polish Grammar

Polish has four main ways to say that something has to happen, and English speakers routinely pick the wrong one — or worse, get the negation backwards and tell someone they mustn't leave when they meant they don't have to. This page sorts out musieć (personal "must/have to"), trzeba (impersonal "one must"), mieć + infinitive (a softer "be supposed to"), and powinien ("should/ought"), and it nails down the single most dangerous trap: nie musieć removes an obligation, it does not impose a prohibition.

musieć — the personal "must / have to"

musieć is the everyday verb for personal obligation. It takes a subject and is followed by a bare infinitive. It is irregular, so learn the present tense as a block:

Person	Form	Meaning
ja	muszę	I must / have to
ty	musisz	you must (informal)
on / ona / ono	musi	he / she / it must
my	musimy	we must
wy	musicie	you must (plural)
oni / one	muszą	they must

Note the spelling: the sz in muszę and muszą, and the nasal ę / ą that mark the ja and oni forms. The stem softens to -si- in the middle persons (musisz, musi, musimy, musicie).

Muszę już iść, bo spóźnię się na pociąg.

I have to go now, or I'll miss the train.

Musisz to przeczytać — naprawdę warto.

You have to read this — it's really worth it.

Dzieci muszą być w szkole o ósmej.

The children have to be at school at eight.

Like English "must/have to," musieć covers both external obligation (a rule forces you) and strong internal necessity (you really need to). In the past it becomes musiałem / musiałam ("I had to"), in the future będę musiał / musiała ("I will have to").

trzeba — the subjectless "one must / it's necessary"

trzeba is where English runs out of single words. It is an impersonal predicate: it has no subject, never conjugates, and is always followed by an infinitive. It states that an action is necessary without saying who must do it — the obligation hangs in the air, addressed to everyone and no one.

Trzeba to zrobić jeszcze dziś.

This needs to be done today. / We have to do this today.

Trzeba iść, bo robi się późno.

We should get going — it's getting late.

Żeby schudnąć, trzeba mniej jeść i więcej się ruszać.

To lose weight, one has to eat less and move more.

English translates trzeba as "one must," "you have to" (the generic you), "we need to," or a passive ("it must be done") depending on context — but Polish needs none of those scaffolds. The form is fixed: present trzeba, past trzeba było, future trzeba będzie.

If you want to say who the necessity falls on, put that person in the dative case. This is the same "dative subject" pattern Polish uses for feelings and states (see Dative subject and feelings):

Trzeba mi odpocząć.

I need to rest. (lit. to-me it-is-necessary to rest)

Trzeba ci się wyspać przed egzaminem.

You need to get a good sleep before the exam.

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The difference between muszę odpocząć and trzeba mi odpocząć is one of framing. Muszę presents the obligation as yours, actively. Trzeba mi presents it as a necessity that has come over you — more detached, almost like "rest is what I need." Both are natural; trzeba mi sounds a touch more reflective.

mieć + infinitive — "be supposed to / be to"

The verb mieć ("to have") followed by an infinitive expresses a softer obligation: an arrangement, a plan, an instruction, or something expected of you. It is the Polish counterpart of English "be supposed to" and the slightly formal "be to."

Mam tam być o piątej.

I'm supposed to be there at five. / I'm to be there at five.

Co mam robić?

What am I supposed to do? / What should I do?

Pociąg miał przyjechać o dziewiątej, ale się spóźnia.

The train was supposed to arrive at nine, but it's late.

The nuance matters: Mam tam być o piątej reports an arrangement (someone expects me there), whereas Muszę tam być o piątej states a hard necessity (I have no choice). In questions, Co mam robić? is the standard, idiomatic way to ask for instructions — far more natural than Co muszę robić?, which sounds like you're complaining about a burden.

powinien — "should / ought to"

powinien expresses weaker obligation — advice, propriety, what ought to be the case. Grammatically it is a strange beast: it behaves like an adjective, so it agrees in gender and number with the subject, yet it also takes personal endings borrowed from the past tense. It is followed by an infinitive.

	masculine	feminine	neuter
ja	powinienem	powinnam	—
ty	powinieneś	powinnaś	—
on / ona / ono	powinien	powinna	powinno
my	powinniśmy	powinnyśmy	—
wy	powinniście	powinnyście	—
oni	powinni	—	—
one	—	powinny	—

The "my" and "oni/one" forms split by the masculine-personal/non-masculine-personal divide that runs through all of Polish: powinniśmy if at least one man is in the group, powinnyśmy for an all-female or mixed-but-no-men group.

Powinieneś zadzwonić do mamy.

You should call your mum. (said to a man)

Powinnam była ci powiedzieć wcześniej.

I should have told you earlier. (said by a woman)

Powinniśmy już wracać.

We ought to be heading back now.

For past obligation ("should have"), Polish adds a past-tense form of być: powinienem był / powinnam była + infinitive, as in the second example above.

The negation trap: nie musieć ≠ "must not"

This is the single most important point on the page, and the one English speakers get wrong constantly. In English, "must" and "have to" share a meaning in the positive but diverge violently in the negative:

You must not go = it is forbidden.
You don't have to go = there is no obligation, but you may.

Polish lines up with "don't have to," not "must not." Nie musieć simply removes the obligation. It can never mean "is forbidden."

Nie musisz iść, jeśli nie chcesz.

You don't have to go if you don't want to. (no obligation)

Nie muszę dziś pracować — mam wolne.

I don't have to work today — I have the day off.

To express prohibition — English "must not / may not" — Polish uses a completely different construction: nie wolno (+ dative) "it is not allowed," or nie możesz "you can't/may not" (see Ability and permission).

Nie wolno ci tam wchodzić.

You must not go in there. (it's forbidden)

Tu nie wolno palić.

You mustn't smoke here. / Smoking is not allowed here.

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Burn this contrast into memory: nie musisz iść = "you don't have to go" (your choice), but nie wolno ci iść = "you must not go" (forbidden). Confusing them can be the difference between releasing someone from a task and forbidding them from leaving.

The same asymmetry hits trzeba: nie trzeba means "there's no need / no need to bother," not "it's forbidden."

Nie trzeba dziękować.

No need to thank me. / Don't mention it.

And nie powinien is the gentle one: "shouldn't / ought not," advice against rather than a ban.

Nie powinieneś tyle pracować.

You shouldn't work so much.

Common Mistakes

❌ Nie musisz palić tutaj.

Incorrect if you mean 'you mustn't smoke here' — this actually says 'you don't have to smoke here'.

✅ Nie wolno tu palić.

You mustn't smoke here. (prohibition)

A classic transfer error: English "you must not" tempts you toward nie musisz, but nie musisz lifts an obligation. Prohibition is nie wolno / nie możesz.

❌ Trzeba idę do lekarza.

Incorrect — trzeba never takes a conjugated verb.

✅ Trzeba iść do lekarza.

One has to go to the doctor. (trzeba + infinitive)

trzeba is impersonal and is always followed by the infinitive, never a finite (conjugated) verb. If you want a real subject, switch to muszę iść do lekarza.

❌ Powinien iść do domu. (said by a woman about herself)

Incorrect — powinien must agree in gender.

✅ Powinnam iść do domu.

I should go home. (woman speaking)

Because powinien declines like an adjective, a woman says powinnam, a man powinienem. Using the bare masculine powinien for "I should" only works if a man is speaking, and even then it's the third-person form, not the first.

❌ Muszę iść do domu? Co muszę robić?

Awkward — sounds like complaining when asking for instructions.

✅ Co mam robić?

What am I supposed to do? (the idiomatic way to ask)

To ask for instructions, Polish reaches for mieć + infinitive (Co mam robić?), not musieć. Using musieć makes the question sound resentful, as if the task were an imposition.

❌ Muszę dwadzieścia lat.

Incorrect — musieć always needs an infinitive.

✅ Muszę iść.

I have to go. (musieć + infinitive)

musieć is a modal: it cannot stand with just a noun or alone. It must govern an infinitive.

Key Takeaways

musieć (muszę, musisz, musi, musimy, musicie, muszą) = personal "must/have to" + infinitive.
trzeba = impersonal, subjectless "one must / it's necessary" + infinitive; the affected person goes in the dative (trzeba mi).
mieć + infinitive = softer "be supposed to / be to"; the natural choice for Co mam robić? "What am I supposed to do?"
powinien = "should/ought," declines like an adjective (gender + number), takes personal endings.
The negation trap: nie musieć / nie trzeba = "don't have to / no need." Prohibition ("must not") is nie wolno (+ dative) or nie możesz — never nie musieć.

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