Learn Finnish: Etsin valkoista sukkaa. - I am looking for a white sock.

In Finnish, adjectives have to match the case of the noun they describe. Because the noun sukkaa is in the partitive case, the adjective valkoinen must also change to its partitive form, valkoista. This matching is called adjective agreement.

How does the basic word valkoinen turn into valkoista?

Finnish words that end in -nen follow a specific pattern. To form the partitive case, you drop the -nen ending and add -sta (or -stä depending on vowel harmony). So, the stem valkoi- plus -sta gives us valkoista.

Why is the noun sukka (sock) in the partitive case (sukkaa) in the first place?

The verb etsiä (to look for) is a partitive verb. This means the thing you are searching for must always be in the partitive case. It makes sense because searching is an ongoing, incomplete action until you actually find the item!

Since it is in the partitive case, does valkoista sukkaa mean "some white socks"?

No, it means "a white sock" (singular). Even though the partitive case is often used to mean "some" with food or liquids, here the partitive form is just a strict grammar rule forced by the verb etsin. If you were looking for multiple socks, you would need the plural partitive form: valkoisia sukkia.