A lot of us get tripped up by words that sound very similar, but they mean different things. And one example of that is the word have and the word "of". You might hear someone say, "I should of drank more coffee this morning." They're saying the word "of", but they mean to say the word "have". Let me explain to you the difference. Essentially, "have" is a verb; it expresses an action. "Of" is a preposition. It expresses a location of one word in relation to another word in a sentence.
So here's an example of how you would use each of them in a sentence the correct way. So for instance, let's look at the sentence, "I should have drank more coffee." In this case we use the word "have" because we need a verb to indicate that we should have drank more coffee. Although sometimes people use the contraction, and that's okay too as long as you write it the correct way. If you want to use the contraction for "should have" you write, "I should've drank more coffee." Now you can hear that "should've" sounds a lot like "should of", which is why often times people write the incorrect word. And the correct way to use the word "of" in a sentence is, "I ordered a cup of coffee." In this case, we need a preposition, and "of" is truly the correct word, so I hope this clears up the difference between "have" and "of" and why our ears can make it a little bit tricky.