Learn Hardcore Finnish: Auto on vakuutettu.

on is the third-person singular present tense of olla (“to be”). Here it functions both as the main verb “is” and as the auxiliary that helps form the perfect passive: on + past passive participle.

What form is vakuutettu?

vakuutettu is the past passive participle of vakuuttaa (“to insure”). In Finnish perfect passive, you combine on with this participle to say “has been insured” or simply “is insured” in a state sense.

Why doesn’t vakuutettu change its ending to agree with auto?

In the perfect passive construction, the participle (vakuutettu) is indeclinable—it never takes case or number endings. The focus is on the action’s result, not on who did it or on the grammatical agreement.

Is this sentence in the passive voice?

Yes. More precisely, it’s the perfect passive, which indicates that the action (insuring) is completed and the car is now in the state of being insured.

How do you ask “Is the car insured?” in Finnish?

Attach the question particle -ko to the verb and invert subject and verb:
Onko auto vakuutettu?
Literally: “Is-Q the car insured?”