Questions & Answers about Olitko sinä eilen kotona?
How do we make a question out of "you were" in this sentence?
The word for "you were" is olit. To turn a statement into a yes/no question in Finnish, you add the question ending -ko to the verb. This makes it olitko.
Could I use olet instead of olit here?
No, because olet means "you are" in the present tense. Since this sentence talks about yesterday (eilen), you must use the past tense form for "you were", which is olit.
Is it absolutely necessary to include the word sinä (you)?
No, it is optional! The verb olitko already contains the "you" ending (-t), so asking Olitko eilen kotona? is perfectly correct and very common in spoken Finnish. Including sinä just adds a little extra emphasis.
Why is the word for home kotona instead of just koti?
Koti is the basic dictionary word for "home". However, when you want to say that someone is currently at home or in their home, you must use the special location form kotona.