How do I insert a linebreak where the cursor is without entering into insert mode in Vim?

For the example you’ve given, you could use rEnter to replace a single character (the space) with Enter. Then, fspace. to move forward to the next space and repeat the last command.

Depending on your autoindent settings, the above may or may not indent the return statement properly. If not, then use sEnterTabEsc instead to replace the space with a newline, indent the line, and exit insert mode. You would have to replace the second space with a different command so you couldn’t use ‘.’ in this case.