We tend to use "write down" for things that we want to remember and refer to later (e.g., a list, a recipe, a phone message, an appointment, an arrival/departure time, etc.). It is used for basic, functional information, like note-taking; whereas an author "writes" a book, he doesn't "write down" a book.

만약 기억하고 뭔가가 있는 싶으다면, "write something down"입니다. If there is something you want to remember... you >>> write it down. << 아! 저것을 기억해야 돼요! >>> I should write it down! <<< (down) 쓰다/썼어요(write/wrote) ... 언제든지 읽을 수 있습니다. To write / can be read anytime. 천천해주십시오. Take your time. 긴급 느낌이 없습니다. (아닙니다) There is no urgent feeling. (저는 책을 썼어요) ... (당신에게 메세지를 썼어요.) I wrote the book. I wrote you a message.

적어놓다 ? - < 그렇게 아닙니다... Brad C.의 대답을 맞지습니다.

안녕하세요~ write 쓰다 write down 받아적다, 적어놓다 - 대충 이래요.