In English, this sounds awkward and unnatural. Is this a common construction in Latin?
Yeah. It's meant to be used in sentences that contrast two different parts... Not this (non), but the other (sed). In the first levels of Latin, Duo uses very simple words, so sometimes it sounds weird but syntactically correct.
Agreed. In English we are more likely to put the verb with the first part. The boy is not writing, but the man. However, in Latin this is quite common.
I'll grant it's an uncommon way of phrasing it in English, but I have heard this construction before in certain contexts.
I recommend "It is not a/the boy but a/the man who writes/is writing". Or perhaps even more naturally in English: "The boy does/is not write/writing but the man does/is."
Probably... but punctuation came along pretty late in the game. Hell, spaces between words was a revolutionary innovation. Ya just gotta keep it loose and kinda go with the flow.
Give more options for good translations on this one: "The boy does not write but the man does"
Write that, wait for the lesson to reject it, and flag it so you can report "My answer should be accepted." That is the only way to bring it to the attention of the course contributors. Simply saying it in here will not reach them.