Learn Croatian: Pijemo kavu ili čaj? - Are we drinking coffee or tea?

The word kava is the base dictionary form. Here, it is the direct object of the verb pijemo (we drink). Feminine nouns ending in -a change their ending to -u in the accusative case, which is the grammatical case used for direct objects.

If kavu changed its ending, why does čaj stay the same?

The word čaj is also a direct object in the accusative case. However, čaj is a masculine noun that refers to a non-living thing (inanimate). For masculine inanimate nouns, the accusative form looks exactly the same as the basic dictionary form.

Where is the word for "we" in this sentence?

It is built right into the verb! The -mo ending on pijemo tells us that "we" are doing the action. Croatian usually drops the pronoun mi (we) unless you want to put special emphasis on who is drinking.

How do we know this is a question if there are no question words?

In Croatian, you can turn a regular statement into a question simply by using a rising tone of voice, which is shown by the question mark in writing. You do not need extra helper words like "do" or "are".