Learn Croatian: U banci sam. - I am in the bank.

The dictionary word for bank is banka. When you are talking about being located somewhere, Croatian uses the locative case, which changes feminine -a endings to -i. Also, there is a special sound rule: when the letter k comes right before that new -i ending, it changes to c. Because of this, banka becomes banci (not 'banki').

Why is the word sam (am) at the very end of the sentence?

The word sam is a type of word called a clitic. Clitics are short, unstressed words that usually need to be in the second position of a sentence. Because the preposition and noun (U banci) work together as a single block of meaning, sam takes the second slot right after it. In a short sentence like this, that happens to be the end!

Where is the word for 'I' (ja) in this sentence?

In Croatian, you can usually leave out subject pronouns like ja (I) because the verb sam already clearly means 'I am'. You would only say Ja sam u banci if you wanted to emphasize that you are at the bank, as opposed to someone else.

How do we know u means 'in' here?