Learn Finnish: Oletteko te nyt bussissa?

Languages often disagree on exactly how to describe locations! While English speakers say they are 'on' a bus or train, Finnish speakers consider themselves to be 'in' the vehicle. So, you use the 'in' ending (-ssa) to make bussissa.

How do I know whether to use -ssa or -ssä for the word bussi?

This relies on vowel harmony. You look at the vowels inside the word itself. The word bussi contains the back vowel u, so it pulls the ending to the back vowel version, which is -ssa.

What does the -ko at the end of Oletteko do?

It is a question particle. You take the verb olette (you are), stick -ko on the end, and put it at the start of the sentence to create a yes/no question: 'Are you?'.

Since olette already means 'you are' (plural), is the pronoun te required?

No, you can safely drop it! Because the verb ending -tte clearly tells us who the subject is, you could just say Oletteko nyt bussissa? Including te is perfectly fine and often done in spoken Finnish or for slight emphasis.