How to break a line in vim in normal mode?

I don't know of a single key command, but a lot of times I do "r" then "Enter" to break a line.

"r" replaces the current character under the cursor without going into insert mode. This may not be what you want if you don't want to replace a character...

put cursor in position and...

  r<Enter>

Try this:

:nnoremap <NL> i<CR><ESC>

then just press Ctrl-J whenever you want to split a line.