Here's a quick list of shortcuts you may use in Desmos. You can find these keyboard shortcuts and more in the help menu in the calculator or by pressing ctrl + / on Windows, command + / on Mac.
Save: ctrl + s
Save As or Rename: ctrl + shift + s
Open (saved graphs): ctrl + o
Undo: ctrl + z
Redo: ctrl + y
New expression: press "enter"
Add a new note: type " in an empty expression
Exponent: can be entered like x^2
Subscript: can be entered like y1 or v_ariable
You can also move around the expression list and within table cells using the arrow keys
√: type "sqrt" (you can also type "nthroot" for cubed roots, etc)
∑: type "sum"
π: type "pi"
θ: type "theta"
∏: type "prod"
A printable version of this page is available for download here: http://s3.amazonaws.com/desmos/Desmos_Calculator_User_Guide.pdf