Learn Finnish: Syy ja seuraus ovat eri asioita. - Cause and consequence are different things.

It is a noun derived from a verb! It comes from the verb seurata (to follow). By taking the stem seura- and adding the result suffix -us, we create a noun that means the outcome of that action. So, the result of following is a seuraus (consequence).

Why doesn't the adjective eri change its ending to match asioita?

The word eri (different) is a special invariable adjective in Finnish. This means it never changes its ending, no matter what case or number the noun it describes is in. Even though asioita is in the partitive plural, eri stays exactly as it is.

Why is asioita in the partitive plural here instead of the nominative plural asiat?

In Finnish sentences where you describe what a plural subject is (A and B are X), the describing noun is put into the partitive plural if it represents an open-ended category, an indefinite amount, or an abstract concept. Because "different things" isn't a fixed, specific group of items, we use the partitive plural asioita.

Can the word syy mean anything else besides "cause"?

Yes, syy is a very common word that also means "reason" or "fault/blame." For example, Se on minun syyni means "It is my fault." However, when paired with seuraus (consequence) like in this specific sentence, it translates best as "cause."