How to go back (ctrl+z) in vi/vim

You can use the u button to undo the last modification. (And Ctrl+R to redo it).

Read more about it at: http://vim.wikia.com/wiki/Undo_and_Redo

The answer, u, (and many others) is in $ vimtutor.

Here is a trick though. You can map the Ctrl+Z keys. This can be achieved by editing the .vimrc file. Add the following lines in the '.vimrc` file.

nnoremap <c-z> :u<CR>      " Avoid using this**
inoremap <c-z> <c-o>:u<CR>

This may not the a preferred way, but can be used.

** Ctrl+Z is used in Linux to suspend the ongoing program/process.