Learn Icelandic: Það er enginn í þessum banka.

þessum is the dative masculine singular form of þessi (this). We use the dative case here because the preposition í (in) takes the dative when describing a fixed location or a state of rest, like being inside a building.

I thought the word for bank was banki. Why is it banka here?

banki is a masculine noun. Because it is part of the phrase "in this bank", it must match the dative case of þessum. For masculine nouns ending in -i (often called weak nouns), the ending changes to -a in the dative case. Therefore, banki becomes banka.

Why is enginn in the masculine form if it just means "nobody"?

When used as a standalone pronoun to mean "no one" or "nobody" in a general sense, Icelandic defaults to the masculine singular form enginn. It is in the nominative case here because it is the actual subject of the sentence (the "nobody" who is in the bank).

Does Það er always mean "There is"?

Yes, when you want to state that something exists or is located somewhere, Icelandic uses Það er (literally "it is"). The word það acts as a placeholder or dummy subject to kick off the sentence, exactly the same way we use "there" in English for "there is" or "there are".