To split a line at the cursor position you can use the usual sequence of i, Enter and Esc. This enters insert mode, adds a new line and then escapes from insert mode.
If your cursor is at a space in English text then you are in luck. You can insert a newline here by replacing the space with a newline character. To do that in Vim, press r and Enter. No need to escape insert mode!
Tried with: Vim 7.4