Learn Finnish: Juon vain mustaa kahvia. - I drink only black coffee.

In Finnish, adjectives must match the form of the noun they describe. The noun here is kahvia, which is in the partitive case (ending in -a). Because the noun is partitive, the adjective musta also has to take the partitive ending -a to match, becoming mustaa. This is called adjective agreement.

Why is kahvia in the partitive case instead of just being the basic word kahvi?

Coffee is an uncountable substance (a liquid). When you are consuming an unspecified amount of a mass noun—like drinking "some" coffee—the word takes the partitive case. If you used the basic object form, it would mean you are drinking one specific, whole portion (like a full cup) rather than coffee in general.

Where is the word for "I" (minä) in this sentence?

It's built right into the verb! The basic verb is juoda (to drink), and it takes the ending -n to become juon (I drink). Because the -n ending makes it crystal clear who is doing the action, you can safely drop the pronoun minä in Finnish.

Why is the word vain (only) placed exactly where it is?

The word vain usually comes immediately before the word or idea it emphasizes. Here, the focus is on the fact that you drink only black coffee (and nothing else), so vain is placed right in front of the phrase mustaa kahvia.