Learn Finnish: Kuka istuu sohvalla? - Who is sitting on the sofa?

The ending -lla is the adessive case, which usually translates to "on" or "at" in English. Because someone is sitting on the sofa, we take the base word sohva (sofa) and attach -lla.

Why is the ending -lla and not -llä?

Finnish uses vowel harmony. The word sohva has the back vowels o and a, so it needs the back-vowel ending -lla. If the word had front vowels like ä or ö, it would use -llä instead.

The English translation says "is sitting", but I don't see a word for "is". Why?

Finnish doesn't have a separate continuous tense like English does. The single verb istuu means both "sits" and "is sitting". You never translate the "is" part separately in these situations!

Why does the verb istuu end in a double u?

The dictionary form of the verb is istua (to sit). When we talk about "he", "she", "it", or kuka (who), we take the stem istu- and double the last vowel. That gives us istuu.