Learn Finnish: Oletko kiinnostunut historiasta? - Are you interested in history?

The word kiinnostunut (interested) always demands that the thing you are interested in is put in the elative case, which uses the -sta or -stä ending. While in English you are interested 'in' something, in Finnish you are grammatically interested 'from' it.

What does the -nut ending on kiinnostunut mean?

It shows that the word comes from the verb kiinnostua (to get interested). The -nut ending turns a verb into a descriptive word (a past participle) describing a state someone is in, so kiinnostunut essentially means someone who has 'gotten interested'.

Why is the question ending -ko added to olet?

To make a yes/no question in Finnish, you add -ko or -kö to the word that is the main focus of the question and move it to the beginning of the sentence. Here, attaching it to olet (you are) creates the basic question 'Are you?'.

How would I ask 'Are you interested in this?' instead of history?

You need to put the word 'this' (tämä) into the exact same case as historiasta. The correct form of tämä with the -sta ending is tästä, so the new sentence becomes Oletko kiinnostunut tästä?