I would suggest thinking of 그러면 more like 'If that's the case' or 'If (that's) so' It comes mainly just at the beginning of sentences like most sentence connectors such as 그리고 and 그러나. If you want a word-for-word for it it's literally "if it is like that (thing)"

그러면, 그래서 can use "and" but 그러면 has mean "then or if you do" knock and the door will open. In this sentence, "and" means 그러면 그래서 can use "so or therefore" So what happened? in this situation, "so" means 그래서