Toggle Normal-Insert Modes
This is a simple setup I use here and I believe it is way easier than using Esc-I combination to go back and forth between modes. This is a toggle based solution. You can go back and forth between Insert and Normal modes via "ctrl+space"
Copy the lines below into your Vim settings file.
"Normal-Insert Toggle
nnoremap <C-space> i
imap <C-space> <Esc>