"제가 메시지를 연필로 적습니다."
Translation:I write a message with a pencil.
Is there are reason why it's 적습니다 and not 씁니다? That has been used as 'to write' in other sentences
쓰다 is used when you need some creativity to write, such as writing a letter or a poem. 적다 is for just writing down something.
I suggest reading a few lessons about it has its quite complex to explain.
Basically, the first marker (subject) bolds the noun and the second (topic) highlights it.
The second also can be used to contrast amongst a number of objects. So it could be, with the topic marker, "I, as opposed to other people, write a message with pencil."
While the first marker could be read as: I write a message with pencil.
You're confusing me.
"I write/copy/take down the message." Here you're emphasizing that you as opposed to other people are the agent of the message getting written, so 제가.
"(As for me,) I write/copy/take down the message." Here you're emphasizing that the message is getting taken down, as that's just the sort of person you are. You're talking about yourself, what you're up to, not so much that you're the one in particular doing it, so 저는. Well, unless it's different in Korean. It does seem to work this way in English and Japanese . . .
으로/로 can also mark instrumental. When used after a place and with a verb of motion, it is "toward," but in other cases it is often translated as "with." https://www.duolingo.com/skill/ko/regular-verbs/tips-and-notes