Learn Finnish: Juotko kuumaa teetä?

In Finnish, adjectives have to match the case of the noun they describe. Because the noun teetä is in the partitive case, the adjective describing it also needs a partitive ending, making it kuumaa.

Why is tea in the partitive case (teetä)?

Tea is an uncountable substance or a "mass noun." When you are doing something to an unspecified amount of a substance, like drinking some tea, it becomes the object of the sentence in the partitive case.

Both words are in the partitive case, so why does kuumaa end in -a but teetä ends in -tä?

This is due to vowel harmony! The word kuuma contains the back vowels u and a, so it takes the back-vowel ending -a. The word tee only has the vowel e, which pairs with front vowels, so it takes the front-vowel ending -tä.

How does juotko mean "do you drink"?

It is built from the verb juoda (to drink). First, it is conjugated for "you" to become juot (you drink). Then, the question particle -ko is attached to the end to turn the statement into a yes/no question.