"Someone is at the door."
Translation:Er is iemand aan de deur.
It’s not really a correct sentence. I, as a native, am not even sure why, but I guess it has to do with the fact that ‘iemand’ is an indefinite pronoun. If you’d say “Robert staat aan de deur”, that would be correct. But with indefinite pronouns like ‘niemand’ or ‘iemand’, “Er staat niemand voor de deur” is correct. But for some reason, ‘iedereen’ doesn’t comply with that rule. “Iedereen staat voor de deur” is correct, without ‘er’.
I honestly have no idea; the word ‘er’ is a very big challenge and I can barely find anything online about it. You will have to learn through experience.
In this case, the order of words wouldn’t make sense. It suggests that ‘iemand’ would be the subject. “Aan de deur zijn” isn’t an expression/a verb, so this sentence isn’t correct. That’s why “er is” is used: There is someone (“er is iemand”) – at the door [location]. This construction with ‘er’ allows this sentence to be correct. Since in English, “to be at the door” is a verb, this sentence has more flexibility in English. ‘Er’ is a hard word! It’s hard to explain as well. Using it flawlessly requires practice and experience.