Learn Finnish: Minun olisi täytynyt ostaa taskulamppu, sillä sähkökatko oli pitkä. - I should have bought a flashlight, for the power outage was long.

Because this sentence uses a verb of obligation (täytyä). In Finnish, verbs expressing necessity require the person doing the action to be in the genitive case. This rule applies across all tenses, so just like 'I must' is Minun täytyy, 'I should have' is Minun olisi täytynyt.

How is the 'should have' structure formed here?

It is built using the conditional form of the verb 'to be' (olisi) followed by the past participle of täytyä (täytynyt). Because it is a verb of obligation, olisi always stays in the third-person singular rather than matching the subject.

Could I use the verb pitää instead of täytyä?

Yes, you could say Minun olisi pitänyt ostaa. The structures olisi täytynyt and olisi pitänyt both mean 'should have' and are interchangeable here. Olisi pitänyt is especially common in spoken Finnish.

Why is taskulamppu in the basic nominative form instead of the genitive taskulampun?