Learn Dutch: Er is iemand bij het station. - There is someone at the station.

In this sentence, er is used just like the English 'there' in 'there is' or 'there are' to show that something exists. If you used daar, it would mean pointing to a specific physical place, like 'over there is someone'.

How do I know whether to use er is or er zijn here?

You look at the subject that follows it! Because iemand (someone) is singular, we use er is. If we were talking about multiple people or things, we would use er zijn (there are).

Why is the preposition bij used here for 'at'?

Bij is often used to mean 'at', 'near', or 'by' a general location. If the person were specifically inside the building, you could say in het station, but bij het station is the most common way to say someone is at the station.

Is iemand always singular?

Yes. Just like the English word 'someone', iemand is always treated as a single person in Dutch. That is why it pairs with the singular verb is.