לרשום = to write meaningless things, or things you don't need to think about too much. such as filling a form, writing down a phone number, a list for the supermarket, etc
לכתוב = to write books, exams, an email, etc`
what is the difference between: "I am writing down this" and I am writing this down?
Because it sounds very wrong and no one would ever say it like that. When using a pronoun it must be in the middle.
It's there now (I can't see how old your comment is in the app, it's now 21 November 2018).