Almost everything is driven by Tab and Enter — you rarely need the mouse.
Tab cycle the current element type (Shift+Tab = back)
Enter finish element, move to the next logical one
Enter on empty fall through (empty Character → Action)
Shift+Enter line break inside the same element
Backspace at start merge into the element above
⌘/Ctrl + 1…7 set element directly
⌘/Ctrl + A, A select element; press again for whole script
⌘/Ctrl + Z / ⇧Z undo / redo (whole document)
⌘/Ctrl + S / O save / open a .fountain file
INT. / EXT. starts a scene heading automatically
. ! @ > force Scene / Action / Character / Transition
( in a speech starts a parenthetical