The following output shows you how using the question mark can help you to work your way through a command and all of its parameters.
Lists all commands available in the current command mode
Lists all the possible choices that start with c
Lists all the possible choices that start with cl
% Incomplete Command
Tells you that there are more parameters that need to be entered
Router#clock ? set
Shows all subcommands for this command. Sets the time and date
Router#clock set 19:50:00 14 July 2003 ?
Pressing the key confirms the time and date configured
No Error message/Incomplete Command message means the command was entered successfully