Learn Croatian: Idem u banku. - I am going to the bank.

In Croatian, the ending of the verb tells you who is doing the action. Idem is the "I" form of the verb ići (to go). Because the verb already shows who is going, you can usually drop the word for "I" (ja) unless you want to add special emphasis.

The word for bank is banka. Why does it change to banku here?

Because you are moving towards a destination. When you express motion to a place, feminine nouns that end in -a (like banka) change their ending to -u. This is called the accusative case, which we use to show the target of our movement.

Doesn't the word u mean "in"? Why does the English translation say "to"?

The preposition u can mean "in" (when you are resting inside a place) or "into/to" (when you are moving towards it). With verbs of motion like ići, u shows that you are going inside the building, so "to the bank" or "into the bank" is the best translation.

Is there a difference between "I go" and "I am going" in Croatian?

No, Croatian doesn't have a separate "-ing" tense like English does. The single word idem covers both "I go" (as a habit) and "I am going" (right now). You can usually tell which one is meant from the context.