Learn Finnish: Meidän piti evakuoida kaupunki tulvan takia. - We had to evacuate the city because of the flood.

In Finnish, sentences expressing obligation or necessity use a special structure. The person who has to do something is put in the genitive case. So, me (we) becomes meidän.

How does piti work here to mean 'had to'?

The verb pitää has several meanings, but when used with a genitive subject, it acts as a necessity verb meaning 'must' or 'to have to'. Piti is its past tense form ('had to'). In these necessity structures, the verb is always frozen in the 3rd person singular form, so it is just piti rather than matching the 'we' subject.

Why is the object kaupunki in its basic nominative form, rather than kaupungin or kaupunkia?

This is one of the trickiest rules in Finnish! Normally, when you do something to a whole object, it takes the genitive form (which would look like kaupungin). However, in a necessity sentence where the subject is already in the genitive case (like meidän), a whole object must be put in the basic nominative form instead. Since the entire city was evacuated, it is simply kaupunki.

Why does tulva (flood) change to tulvan before takia?