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Backups. If something is worth saving, it's worth saving in at least two places. And not all in the same room!
Disks fail, computers are stolen, keyboards take coffee baths, houses burn and even clouds can lose your files. Plan ahead 
And beyond copy/paste and undo ... if you can touch-type and plan on doing more than a little bit of typing, learning at least some of the ancient emacs-style key shortcuts can save a lot of time! I was recently shocked that my kids (in their 20s, Mac users since they were infants) didn't even know they existed
They let you navigate and edit without having to leave the keyboard and use the mouse/trackpad. And they work in almost all editing situations in macOS. Even with a trackpad, I find I'm using them all the time. E.g. it's faster to position the cursor and hit control-d to delete a character than to carefully drag over a single character and then hit delete.
They all use the control-key ... so if you're not comfortable hitting control with a pinky finger, you can stop reading now.
But ... some of the more useful ones:
control-d delete the next character
control-a go to beginning of the line
control-e go to the end of the line
There are the same as the arrow keys, but faster to hit:
control-f go forward a character
control-b go backward a character
control-n go to next line (trying to leave x-coordinate the same)
control-p go to previous line (trying to leave x-coordinate the same)
Using the kill buffer:
control-k delete from cursor to end of line (*)
control-y yanks back the text deleted by control-k (inserts at current cursor position)
(*) if the line is empty, it deletes the line (the linefeed). Else it deletes up to but not including the linefeed. And you can hit control-k several times to delete a small number of lines. Each time you control-k it adds the deleted text to the end of the "kill" buffer. Doing another control-k deletes more and adds it to the end of the buffer. Moving the cursor and doing a control-k in a new "place" will discard the kill buffer and replace it with the newly killed text. Note the kill buffer is not the same as the normal copy/paste buffer!
Ok, probably too much info. But I feel more people should know about the editing shortcuts too 