Unusually for a modern application, Flash demands that you learn to use the function keys at the top of your keyboard. (On a Mac, these rarely-used keys are typically reserved for changing screen brightness, sound volume, and the like. You can change this default setting in the System Preferences.) The essential commands are:

( Create )
F7: make a new empty frame.
F6: duplicate the current frame.
F5: extend the current frame.

( Preserve )
F8: convert the current selection into a Symbol.
Command – F8 (Mac) or Control – F8 (PC): make a new empty Symbol.

( Destroy )
Shift – F5: delete the current frame, and close the gap on the timeline.
Shift – F6: delete the current frame, and leave the rest of the timeline in place.