When we talk, it is common to use the words could, should and would. We naturally contract these words when we talk to make conversations flow better. For example, I would’ve gotten another color, but this is all they had.
The problem comes in when people try to write it out. They hear it in their heads and write would of instead of would’ve. It works the same way with could’ve and should’ve.
The old adage, Could’ve, Should’ve, Would’ve… is correctly written here. This is because each of these words is the contraction for could have, should have, and would have.
There is no such phrase as could of or would of or should of. Yes, it sounds like it, but it is really have. This also occurs with must’ve, which is a contraction of must have, not must of. Of is not a verb, while have is. Just try to conjugate “of” and you’ll find you can’t, while you can say I have, He has, She had, etc.
Wow, I could’ve had a V-8.
I should’ve double checked my spelling before I handed in my essay.
I would’ve chosen an apple instead.
Why not commit this to memory so your next letter or memo to the boss doesn’t contain this nonexistent phrase? 
John and Toni Rakestraw are the owners of Rakestraw Book Design. Toni is the editor; she keeps all those words in line. John is the voice of the company. He can often be found hosting Google+ Hangouts on writing and issues writers face online. Archives for his shows can be found on YouTube.
