Learn Swedish: Att röka i hissen är olagligt här. - To smoke in the elevator is illegal here.

The prefix o- is a very common way to create a negative or opposite meaning in Swedish, similar to un-, in-, or il- in English. It attaches to the base adjective laglig (legal) to create olaglig (illegal).

Why does olagligt end with a -t here?

In this sentence, the adjective describes the entire action of the infinitive phrase Att röka i hissen (To smoke in the elevator). In Swedish, whenever a verb phrase or a general action is the subject of the sentence, the adjective describing it must take the neuter -t form.

What are the building blocks of the base word in olagligt?

The root word is the noun lag, which means law. Adding the suffix -lig turns it into the adjective laglig (lawful or legal). Finally, adding the negative prefix o- makes it olaglig (illegal).

Why is it att röka and not just röker?

Because smoking is the subject of the sentence (the thing that is illegal). Just like in English where we say To smoke is illegal or Smoking is illegal, Swedish uses the infinitive marker att with the infinitive verb röka to turn the action into a noun-like phrase.