There is no such phrase as “could of” so why do so many people say and write it? The problem lies when you shorten ‘could have’ to ‘could’ve’.
Try saying it. It sounds like ‘could of’ doesn’t it? Those unfamiliar with the English language or even those who have just grown up saying it this way, can therefore be forgiven for thinking that ‘could of’ is correct.
Now this would be a very short lecture if I simply stated that there was no such phrase as ‘could of’ and therefore no-one should use it. Obviously those people who do get confused thought they were using it in its right context, so let’s explain a little further about this common grammar mistake.
‘Could’ is plainly the past tense of the modal verb ‘can’. A modal verb is something that expresses necessity or possibility. So according to the Oxford Dictionary, ‘can’ means: “be able to”.
“I can run fast”
“he can speak Italian”
“you can move the clock if you want to”
“Can I have a drink please?”
All of the above examples illustrate permission, a request or an ability to do something.
So if the word can represents the ability to do; ‘could’ represents past versions, of having done something, gained permission or made a request.
“I could run fast”
“He could have moved the clocks if he wanted to”
“She told him he could have a drink”
So the past word of ‘could’ is correctly paired in these instances with the past word of ‘have’.
Yet many people replace ‘have’ with ‘of’. If it makes you feel any better, the error was apparently recorded as early as 1837 and is still being used frequently today!
Of is used when expressing a relationship between two items, whether that be in mathematics or elsewhere.
Have is used to project ownership, such as that of an item, an experience, a skill, etc.
Both words have completely different meanings which will help you to put your phrase in context and understand why there could never be a correct usage for ‘could of’. Imagine saying “I of, she of, it of, he of” – it doesn’t make sense does it? “I have, she has, it has, he has” does make sense.
“I could of told you that!”
“She could of had the pie herself”
Replace ‘could of’ with ‘could’ve’ and it makes much more sense!
Simply remember this, could is a helping verb and needs to be with another verb of which ‘have’ is. Does the word of look like a verb to you?
Now picture the words ‘could’ and ‘have’ as two fat ladies walking side by side, matching each other perfectly and then picture of as a skinny lady trying to push in.
This visual aid should help you.
So yes, I could’ve stuck to a quick lecture but I hope my in-depth explanation helps better!