Learn Finnish: Mitä te ajattelette tästä ajatuksesta? - What do you think about this thought?

ajatus (a thought or idea) is built from the root of the verb ajatella (to think) using the -us suffix. This suffix creates a noun representing the result of the action. Notice that the double consonant -ll- from the infinitive verb disappears during this specific derivation.

Why do we use the elative case tästä ajatuksesta (about this thought)?

The verb ajatella (to think) requires the elative case (-sta/-stä) when you are stating what you are thinking or having an opinion about. Both the pronoun tämä and the noun ajatus must be in this case, becoming tästä ajatuksesta.

Why is "what" translated as mitä instead of mikä?

Mitä is the partitive form of mikä (what). The verb ajatella is a cognitive verb, and verbs dealing with continuous thoughts, processes, and feelings usually require their object to be in the partitive case.

Why is there a double tt in ajattelette, but only one t in ajatella and ajatus?