I would like to break a line (at the location of the cursor) in to two lines without leaving normal mode (entering insert or command-line mode). Is this possible?
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...