Learn Hardcore Finnish: Mitä tapahtui eilen illalla museossa? - What happened last night at the museum?

Mitä is the interrogative pronoun “what.” In Finnish questions you use the partitive form of mikä (“which/what”), which is mitä. Even though tapahtua (“to happen”) is intransitive (no object), the question word still appears as mitä.

Why is the verb tapahtui used here and what tense is it?

Tapahtui is the past tense, third-person singular form of tapahtua (“to happen”). So Mitä tapahtui…? literally means “What happened…?”

What role do eilen and illalla play, and why are they both needed?

Eilen is an adverb meaning “yesterday.”
Illalla is the adessive case of ilta (“evening”), meaning “in the evening.”
Together eilen illalla specify “yesterday evening.” You need both if you want to emphasize when something happened.

Why is museossa used instead of just museo?

Museossa is the inessive case (stem +museo + -ssa), meaning “in the museum.” The suffix -ssa always marks “inside” a location.

Why doesn’t this sentence have a subject like it or there?

Finnish often uses impersonal constructions for verbs like tapahtua. The verb form tapahtui already implies a third-person subject (“it happened” or “there happened”). No separate subject pronoun is required.

Can I change the word order, for example Museossa eilen illalla mitä tapahtui?

You can rearrange elements in Finnish for emphasis, but the default question order is:

Question word (Mitä)
Verb (tapahtui)
Time (eilen illalla)
Place (museossa)
Moving museossa to the front sounds unusual unless you intentionally stress location.

How would I ask “What didn’t happen at the museum yesterday evening?”

Use the negative past form ei tapahtunut and keep mitä in partitive:
Mitä ei tapahtunut eilen illalla museossa?

Are there articles like the or a in Finnish before museossa?

No. Finnish has no articles, so museossa can mean “in a museum,” “in the museum,” or “in museums” depending only on context.