Learn Finnish: Kissa on sisällä tai ulkona. - The cat is inside or outside.

In Finnish, there are two words for "or". You use tai in regular statements like this one. You only use vai when asking a direct question where the listener must choose between options (like "Is the cat inside or outside?"). Because this is a statement of fact, tai is the correct choice.

Why does the sentence just say Kissa? Where is the word for "the"?

Finnish doesn't have articles like "a" or "the". Whether a noun means "a cat" or "the cat" depends on the context and word order. When a basic sentence starts with the subject like Kissa on..., it is usually definite, meaning we translate it as "The cat".

The words for "inside" (sisällä) and "outside" (ulkona) have different endings. Why is that?

Historically, these words were built using different grammatical location endings, but you don't need to worry about the rules for them right now. At the A1 level, it is easiest to just memorize sisällä and ulkona as fixed vocabulary words meaning "inside" and "outside".