Learn Finnish: Nukutko sinä teltassa? - Are you sleeping in the tent?

This is because of consonant gradation. Nukkua is a Type 1 verb. When we conjugate it for "you" (sinä), we add the ending -t. Adding this ending closes the syllable, which triggers a change in the stem: the strong double -kk- weakens to a single -k-. So, nukkua becomes nukut before we even attach the question ending.

What does the -ko at the end of nukutko do?

The -ko (or -kö) ending is a question particle. When you want to ask a yes/no question in Finnish, you take the word you are asking about, put it at the very beginning of the sentence, and add this ending. Here, it attaches to the conjugated verb nukut (you sleep) to make nukutko (are you sleeping?).

Why is "in the tent" spelled teltassa instead of telttassa?

Just like the verb, the noun teltta (tent) undergoes consonant gradation. When we add the "in" ending (-ssa), it closes the final syllable. This causes the strong double -tt- from the dictionary form to weaken to a single -t-, resulting in teltassa.

Since the verb already says who is sleeping, do I have to include the word sinä?