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Insert a space after the cursor:
a<space><esc>
Before:
i<space><Esc>
Replace the character under the cursor with a space:
r<space>
You could create a macro. To create a macro type q followed by any letter in the normal mode. Thus to create a macro with the name x you would type qx. Then all you need is to enter into the insert mode type space and escape back to the normal mode. Once you are done type q to save the macro. From here every time you want to insert a space in the normal mode type @x.
The keystrokes: qxi <Esc>q