Well, I was trying to update a list of IPs for work the other day and realized there are iMac keyboards out there without an INSERT key, which makes editing via the VI editor pretty tough! Turns out there's a simple fix for it!

 
Open your file for editing via Terminal.
 
Then,
 
Type the letter 'a' or 'i' !
 
Yea, I know. Pretty simple. However, once you open a file via the VI editor, there are a few hotkeys that can help you out!

For a quick reference to other hotkeys and other useful stuff, refer to this article at Admin's Choice!
[Link]