Finnish has several ways to say "must," and one of the most characteristic — and most formal — is the frame olla + present passive participle: on tehtävä "must be done," on maksettava "must be paid," on mentävä "must go." It is built directly on the present passive participle -tava/-tävä, and it is the register you meet in instructions, contracts, signs, and regulations. Where everyday speech says täytyy, a printed rulebook says on tehtävä.
The frame: on + -tava/-tävä
The construction is fixed. The verb olla stays in the invariable third-person singular (on in the present, oli in the past), and after it comes the -tava/-tävä participle built on the passive stem. The whole thing reads "is to-be-X-ed," which Finnish uses to mean "must be X-ed."
Auto on pestävä ennen matkaa.
The car must be washed before the trip.
Lasku on maksettava huomenna.
The bill must be paid tomorrow.
Here there is no expressed doer — nobody in particular "must," it is simply the case that the thing has to happen. That impersonality is exactly why the frame suits rules and instructions: a sign that reads Ovi on pidettävä suljettuna "The door must be kept closed" addresses no one and everyone at once.
Ikkunat on suljettava yöksi.
The windows must be closed for the night.
Nämä lomakkeet on täytettävä huolellisesti.
These forms must be filled out carefully.
Building the participle: follow the passive stem
The -tava/-tävä form is the passive present with -an/-än stripped off and -va/-vä added, so its -t- versus -tt- split and its vowel simply mirror the passive. Get the passive right and the necessive falls out.
| Verb | Passive present | Necessive form | Meaning |
|---|---|---|---|
| tehdä (do) | tehdään | on tehtävä | must be done |
| maksaa (pay) | maksetaan | on maksettava | must be paid |
| mennä (go) | mennään | on mentävä | must go |
| ottaa (take) | otetaan | on otettava | must be taken |
| pestä (wash) | pestään | on pestävä | must be washed |
Harmony gives back-vowel -tava (maksettava, otettava) but front-vowel -tävä (tehtävä, mentävä, pestävä). Writing mentava or tehtava is a spelling error, not a stylistic choice.
Personalizing it: the genitive subject
The bare frame is impersonal, but you can name who is under the obligation by putting that person in the genitive, exactly as with täytyy and pitää (see necessive sentences). The genitive subject sits at the front; the participle does not change.
Minun on mentävä nyt.
I have to go now.
Sinun on maksettava tämä lasku itse.
You have to pay this bill yourself.
Meidän on tehtävä päätös ennen perjantaita.
We have to make a decision before Friday.
The object goes in the nominative
Here is the trap that catches every learner. In a normal sentence a completed, countable object takes the accusative -n (ostan auton "I buy the car"). But in the necessive — as in every subjectless "must" construction in Finnish — that total object appears in the nominative, with no -n.
Minun on ostettava uusi auto.
I have to buy a new car.
Hänen on otettava lääke kaksi kertaa päivässä.
He has to take the medicine twice a day.
Auto, not auton; lääke, not lääkkeen. The one exception is personal pronouns, which keep their special accusative -t: Minun on autettava sinua uses partitive sinua (because auttaa governs the partitive), but Minun on herätettävä hänet keeps hänet "him/her."
When to reach for it: register
On tehtävä is (formal), at home in written and official language. In speech and casual writing, Finns overwhelmingly prefer täytyy or pitää.
| Register | "I have to go" |
|---|---|
| (formal / written) | Minun on mentävä. |
| (neutral / everyday) | Minun täytyy mennä. |
| (colloquial) | Mun pitää mennä. / Mun täytyy lähtee. |
Choosing on mentävä in casual chat sounds stiff; choosing täytyy in a legal text sounds loose. Match the frame to the register.
Keep three lookalikes apart
The word tehtävä wears three hats. Do not let them blur:
| Form | Function | Meaning |
|---|---|---|
| on tehtävä | necessive (must + passive participle) | must be done |
| tehtävä | noun (from the same participle) | a task, an exercise |
| on tehty | past passive participle | is / has been done |
Työ on tehtävä = "the work must be done" (still ahead of you); työ on tehty = "the work is done" (behind you). Swap them and you say the opposite.
Common Mistakes
❌ Minun on ostettava uuden auton.
Incorrect — the total object goes in the nominative in the necessive: uusi auto.
✅ Minun on ostettava uusi auto.
I have to buy a new car.
❌ Lasku on maksettu huomenna.
Incorrect — maksettu is the past passive 'paid'; 'must be paid' is the necessive maksettava.
✅ Lasku on maksettava huomenna.
The bill must be paid tomorrow.
❌ Minun on mentava nyt.
Incorrect — mennä is a front-vowel verb, so harmony gives -tävä: mentävä.
✅ Minun on mentävä nyt.
I have to go now.
❌ Ikkunat ovat suljettava yöksi.
Incorrect — the auxiliary stays singular on, even with a plural subject.
✅ Ikkunat on suljettava yöksi.
The windows must be closed for the night.
❌ Minä on tehtävä työ.
Incorrect — the personal subject is genitive, not nominative: minun on tehtävä työ.
✅ Minun on tehtävä työ.
I have to do the work.
Key Takeaways
- The frame is olla (invariable on/oli) + present passive participle -tava/-tävä = "must be X-ed."
- The participle follows the passive stem: tehdään → tehtävä, maksetaan → maksettava, mennään → mentävä.
- Name the obliged person with a genitive subject: minun on mentävä.
- A total object goes in the nominative (uusi auto, not uuden auton); only pronouns keep their accusative -t.
- It is (formal / written) — everyday speech uses täytyy or pitää instead.
- Distinguish necessive tehtävä "must be done," the noun tehtävä "task," and tehty "done."
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- Present Passive Participle -tava/-ttavaB2 — The -tava/-ttava participle: 'to be X-ed / X-able', or in the on-frame 'must be X-ed' — syötävä 'edible', tehtävä 'a task', auto on pestävä 'must be washed'.
- Necessity: täytyy, pitää, on pakkoA2 — The necessive construction for 'must / have to': täytyy, pitää, on pakko — genitive subject, a frozen 3sg verb, a first infinitive, and a nominative object.
- The Genitive Subject: minun täytyyA2 — Why the necessive subject is genitive, not nominative — minun täytyy, opiskelijan täytyy — an impersonal verb frozen at 3sg with an oblique experiencer.
- The Necessive Sentence: minun täytyyA2 — How Finnish expresses obligation — a genitive subject, a frozen 3sg necessive verb, and an infinitive: Minun täytyy mennä. Objects go nominative, negation uses tarvita.