Learn Finnish: Oikeudenkäynti alkaa pian, sillä poliisi syyttää häntä. - The trial begins soon, because the police accuse him.

While both mean because, sillä is a coordinating conjunction (closer to the word for in English). The main practical difference is that a sillä clause can never start a sentence. You cannot say Sillä poliisi syyttää häntä, oikeudenkäynti alkaa pian, but you could do that if you used koska. Sillä also sounds slightly more formal.

Why is the pronoun häntä in the partitive case?

The verb syyttää (to accuse) is a partitive verb. It always requires its object to be in the partitive case, which is why we use häntä instead of the accusative total object form hänet.

In English we treat the police as plural, but why is the Finnish verb syyttää singular here?

In Finnish, the word poliisi is grammatically a normal singular noun when referring to the police as an institution (or to a single police officer). Because the subject is singular, it takes the third-person singular verb syyttää rather than the plural syyttävät.

What is the long word oikeudenkäynti made of?