One shortcut that I use all of the time on a Mac is taking screenshots. There are many times you need to take a screenshot of something you see on your screen and paste it into an email, Word, etc.
To do this, make sure the screen that you’d to take a screenshot of is at the forefront. Then use the following keys (hold each one down, one after the other)
Command + Control + Shift + 4
This will bring up a cross-hair symbol. Click and hold your mouse / trackpad while dragging it to cover the area that you want to include in your screenshot. Once you release the mouse button, the area you selected will be copied into memory.
You can then use the normal “Command + V” combo to paste the screenshot wherever you’d like!