Learn Icelandic: Sitjið þið inni? - Are you sitting inside?

In Icelandic, yes/no questions are made by putting the verb first, followed by the subject. Since this means 'Are you sitting...?', the verb sitjið swaps places with the pronoun þið.

How do we get the form sitjið from the verb sitja?

The subject here is þið (you, plural). In the present tense, the þið form of almost every verb ends in -ið, no matter which verb group it belongs to.

Could I use þú instead of þið if I am talking to one person?

Yes, but you would need to change the verb to match. The sentence would become Situr þú inni? because situr is the correct present tense form for þú (you, singular).

Why do we use inni instead of inn?

Icelandic has different words for location (being somewhere) and direction (moving somewhere). Inni means you are already located inside, safely sitting. If you were actively walking inside, you would use the directional word inn.