Learn Finnish: Poika ei siivoa huonetta aamulla. - The boy does not clean the room in the morning.

Because this is a negative sentence (ei siivoa). In Finnish, the object of a negative verb is always in the partitive case. The base word is huone, and words ending in -e take the -tta ending to become huonetta.

Why is the verb just siivoa?

In negative sentences, the main verb drops its personal ending. The base verb is siivota (to clean). In a positive sentence, 'he cleans' would be hän siivoaa, but after the negative word ei, we only use the base stem form siivoa.

Why does aamulla have the -lla ending?

The -lla ending usually means 'on' or 'at' something (the adessive case), but it is also used with times of the day. Adding it to aamu (morning) is the standard way to say 'in the morning' in Finnish.

Why isn't there a word for 'the' in the Finnish sentence?

Finnish does not have articles like 'a' or 'the'. You just use the noun itself, and whether it means 'a boy' or 'the boy' is understood from the context. When a word like poika is at the start of the sentence, it usually implies 'the boy'.