on oltava

The on tehtävä construction is a 'weighty' alternative to 'pitää' or 'täytyy'. It is the standard for official instructions and legal requirements.

1. Formation:
Take the passive stem of the verb and add the passiivin preesensin partisiippi (also known as the tava-partisiippi) ending -va/-vä. Note that for Verb Types 1, 4, 5, and 6, this results in a double -tt- (e.g., lukea → luetaan → luettava). Verb Types 2 and 3 take a single -t- (e.g., syödä → syödään → syötävä). This participle is also frequently used as an adjective meaning '-able' (e.g., syötävä - edible). For example:

• tehdä (to do) → tehdään → tehtävä
• maksaa (to pay) → maksetaan → maksettava
• olla (to be) → ollaan → oltava

2. The Subject and Tense:
The person who must act is usually in the genitive (minun, sinun, auton). However, the subject is frequently omitted in generic or passive-style instructions (e.g., On muistettava, että... - It must be remembered that...). When there is no genitive subject, the object usually fronts to the topic position (e.g., 'Auto on pestävä'). Intransitive verbs also work perfectly in this construction (e.g., 'Minun on lähdettävä'). The auxiliary verb 'olla' stays in the 3rd-person singular (on, oli, olisi) and never agrees with a plural subject or object. The participle itself also does not take a plural ending (e.g., 'Laskut on maksettava', not 'ovat maksettavat'). For past necessity, use oli: meidän oli mentävä. The conditional olisi gives a conditional obligation or softened recommendation: Meidän olisi mentävä (we should / would have to go).

3. The Object:
This pattern follows specific necessive object rules. A total object is in the nominative (basic form) if it is a singular noun, or the nominative plural (-t) if it is a plural noun. These special object rules apply regardless of whether the genitive subject is present or omitted (e.g., 'Auto on pestävä'). However, personal pronouns (minut, sinut, hänet, meidät, teidät, heidät) and the word kenet must remain in the accusative (-t form). Partial objects always take the partitive.

4. Negation:
For ordinary lack of obligation, use ei tarvitse: Laskua ei tarvitse maksaa tänään (the bill does not have to be paid today). For a prohibition, use ei saa: Tänne ei saa pysäköidä (parking here is not allowed). Direct negative ei ole tehtävä also occurs in formal or legal writing, often meaning 'is not to be done/regarded': Tätä ei ole pidettävä ennakkotapauksena (this is not to be regarded as a precedent). Context controls its scope; do not treat it as an ordinary interchangeable replacement for ei tarvitse.

5. Logical Necessity:
In formal or written Finnish, on oltava is the standard way to express logical deductions or strong certainty (e.g., 'It must be true' → Sen on oltava totta). Everyday alternatives include täytyy olla; on pakko olla can emphasize an unavoidable conclusion.