Questions & Answers about Mitä sinä haluat tehdä aikuisena?
How do we get the form aikuisena from the basic word aikuinen?
The word for adult is aikuinen. In Finnish, words ending in -nen change that ending to -se- before you attach most grammar endings. Then we add the essive case ending -na to express "as a". So the stem aikuise- plus -na gives us aikuisena (as an adult).
The essive case is for "temporary" states, but being an adult lasts a long time. Why is it used here?
In Finnish, the essive case is used for stages of life and roles, regardless of exactly how long they last. Phases like "as a child" (lapsena), "as a student" (opiskelijana), or "as an adult" (aikuisena) always use this case because they describe a role you step into or a phase you go through, rather than what you intrinsically are.
Why does the sentence start with Mitä instead of Mikä?
Mitä is the partitive form of mikä (what). Here, it is the object of the verb tehdä (to do). Because the action of "doing" something here is an open-ended, general activity, the object requires the partitive case. You would use mikä when asking for the subject, like "What is this?" (Mikä tämä on?).