Learn Finnish: Poliisin piti varautua myrskyyn. - The police had to prepare for the storm.

In Finnish, sentences expressing necessity or obligation require the person or thing that 'has to' do something to be in the genitive case. Because this sentence uses the obligation verb piti (had to), the subject poliisi becomes poliisin.

Doesn't the verb piti usually mean 'liked'?

Yes, the basic verb pitää means 'to like' when used with the elative case (ending in -sta/-stä). However, when paired with a genitive subject like poliisin and another verb in its basic dictionary form like varautua, it acts as an obligation verb meaning 'to have to'. Piti is its past tense form, meaning 'had to'.

Could I use täytyi instead of piti in this sentence?

Absolutely. Poliisin täytyi varautua myrskyyn means the exact same thing. Both pitää and täytyy are extremely common verbs for obligation, and their past tense forms (piti and täytyi) are entirely interchangeable in this context.

Why is the word for storm, myrsky, in the illative case (myrskyyn)?

The verb varautua (to prepare for or to brace for) dictates the case of the word that follows it. It requires the thing you are preparing for to be in the illative case (the 'into' case). You can think of it as preparing yourself 'into' the upcoming storm.