There is not a specific rule for it but "som" is ALWAYS used as a conjunktion to something, it can be translated into these English words "as/that/who" BUT remember you have to use it as a link to something, example; who is he? = hvem er han? here you use "hvem" and NOT "som" I don't think there is anybody here who likes him = jeg tror ikke der er nogen her som kan lide ham. here you can use "som" "der" would normally be translated into "there" or "that" but could also be translated into "who" IF you use it as a pronoun, example; it is important we know who did it = det er vigtigt vi ved hvem der gjorde det