How To Change XMonad's Modifier Key

All XMonad shortcuts use the modifier key modMask, which by default is bound to the left Alt key. Often however, we need another key.

XMonad has a so-called “modKey”. This is just one of the keyboard’s modifier keys ➡ Wiki modifier key chosen to be involved in all XMonad’s built-in shortcuts to not conflict with other shortcuts of our operating system.

This “modKey” appears in shortcut configurations as modMask in

((modMask .|. shiftMask, xK_Return), spawn "xterm") 

in native Haskell notation or as M in

("M-S-<Enter>", spawn "xterm")

in an Emacs-like “EZ” notation. Both definitions represent a shortcut that opens a new terminal window when we press Mod + Shift + Enter at the same time.
More: In-depth XMonad shortcuts
See: XMonad shortcuts cheat sheet
See: XMonad shortcuts source code
See: The best editor: (Doom) Emacs

Change the modifier key

modMask usually is the left Alt key, but to avoid conflict with other apps like Emacs, that also rely heavily on that key, many set it to the windows key:

-- myConfig :: XConfig
myConfig = def
  { 
  ..
  , modMask = mod4Mask -- windows key
  ..
  }

With this simple change we have “moved” all shortcuts to other key combination that hopefully don’t conflict with existing apps anymore. Of course we can choose any other modifier key.
More: In-depth XMonad shortcuts

$ xmodmap
..
mod1        Alt_L 
mod2        Num_Lock 
mod3
mod4        Super_L,  Super_R
mod5        ISO_Level3_Shift