This might be blindingly obvious for some readers but it’s something I didn’t know (or, actually, I knew about in the back of my mind but never really used) —
When writing documents or emails you might be used to highlighting some text, then clicking and dragging it to a new position. However, if you hold down Option (Alt), you’ll copy it instead.
This can be very useful if you’re replying to an email. You can select phrases in the original email and copy them to where you’re writing, in order to quote and respond to them. Once the text is dropped into place it will still be indented at the same quote level.