These are two shortcuts I use every day. The first is CTRL-Z - as always, press and hold the ctrl key the tap and release the z key. This undoes the last change you made. Most programs support this and allow you to correct an error. The range of corrections is quite wide: from deleting text, through typing additional text to pasting.
Have a go at undoing: create a Word document or e-mail and type some text into it. Then make some changes - such as pasting additional text or moving text or deleting text. Following each change, type ctrl-z - the last change you made is reversed.
Most editors will support multiple consecutive uses of ctrl-z - i.e. the previous changes are reversed.
Have a go at multiple undoing: make two or three consecutive changes (remember what they were!) then type ctrl-z repeatedly to see if the changes are undone.
The second shortcut is ctrl-a - this selects the entire file. It is very useful when using Notepad as an intermediary to remove formatting as discussed a few days ago. After pasting the selected text into Notepad. Ctrl-a will select it all before copying again. So to use Notepad quickly, copy the text from the source, switch to Notepad then, in sequence: ctrl-v, ctrl-a, ctrl-c. This is paste, select all, copy.
Have a go at copying through Notepad: copy some text, perhaps from this web page. Switch to Notepad then ctrl-v, ctrl-a, ctrl-c. Then switch to the destination editor and paste (ctrl-v).
By the way, in case you hadn’t worked it out, ctrl is short for control!