[Xmonad] Switching workspace ...

[kg]

Thank you for your speedy response .
> when you start XMonad every workspace is empty. Did you try opening
> something (an xterm, for instance)? What happened? 
I can open anything client , for example xterm.
I open a client on the first screen, I change the focus to the second 
screen, and I open a second Xterm on this screen.
I can change a client between screens.

If I have understood the tutoriel, there is a workspace on each physical 
screen.
I don't understand why I can't change the focus with mod-1, mod-2 
whereas I can change with mod-e , mod-w ...