Learn Finnish: Ovatko he junassa vai bussissa? - Are they on the train or on the bus?

In English, we usually say we are "on" large public vehicles like trains and buses. In Finnish, you focus on the fact that you are physically inside them, so you use the inessive ("in") ending -ssa or -ssä. You are literally saying "in the train" (junassa) and "in the bus" (bussissa).

Why do both junassa and bussissa take the -ssa ending instead of -ssä?

This is because of vowel harmony. The word juna contains the back vowels 'u' and 'a', so it takes the back vowel ending -ssa. The word bussi contains 'u' and 'i'. Since 'i' is a neutral vowel, the back vowel 'u' takes charge and dictates the harmony, making it bussissa.

Why use vai for "or" here? Could I use tai?

You must use vai here because this is a question offering two mutually exclusive choices (they are either on the train OR the bus). The word tai also means "or", but it is used in regular statements, not in direct questions where the listener has to pick one option over the other.

What does the -ko attached to the verb ovat do?

Adding the particle -ko (or -kö) to a verb turns it into a question. The verb ovat means "they are", so ovatko becomes "are they?". Even though this sentence asks you to choose between two options rather than just answering "yes" or "no", it still relies on this standard question structure at the beginning of the sentence.