Learn Finnish: Onko kissa tuvassa vai ulkona? - Is the cat in the cabin or outside?

The dictionary word for 'cabin' is tupa. When we add the 'in' ending -ssa, it closes the final syllable (tu-vas-sa). This triggers consonant gradation, causing the single p to weaken and change into a v. That is why tupa becomes tuvassa.

Why use vai for 'or'? Can I use tai instead?

In Finnish, there are two common words for 'or'. You must use vai in questions where you are asking someone to choose between two mutually exclusive alternatives (like being inside the cabin versus outside). You use tai in regular statements. Since this is an either/or question, it has to be vai.

Why does the sentence start with Onko instead of just On?

The basic verb is on (is). To ask a yes/no question, Finnish attaches the question particle -ko to the verb and moves it to the very front of the sentence. This is the standard way to create an 'Is...?' question.

Does the word ulkona also have a location ending like tuvassa?