Learn Finnish: Asianajaja puolustaa naista, sillä hän on viaton. - The lawyer defends the woman, for she is innocent.

Both mean "because," but sillä is a coordinating conjunction (like "for" in English), whereas koska is a subordinating conjunction. The main practical difference is that a sillä clause can never start a sentence. You cannot say "Sillä hän on viaton, asianajaja...", but you could do exactly that with koska.

Why is the woman (naista) in the partitive case?

The verb puolustaa (to defend) is a partitive verb. It expresses an ongoing action directed at someone without a definitive endpoint, so its object must always take the partitive case.

Does hän grammatically refer to the lawyer or the woman?

Common sense tells us it means the woman. However, in strict written Finnish, hän usually refers back to the subject of the previous clause (the lawyer). To make it grammatically crystal clear that "she" refers to the object (the woman), formal Finnish often uses the pronoun tämä instead of hän.

How is the word viaton (innocent) formed?

It is built from the noun vika (fault, defect, or guilt) and the suffix -ton, which means "without" (similar to the English suffix "-less"). So viaton literally translates to "faultless" or "without guilt".