“Should of” is grammatically incorrect in English. The correct phrase is “should have”.
Why do people say “should of” if it is incorrect?
When people say “should have” in English, it is often contracted to “should’ve”. This “‘ve” sound is very similar to “of”. For this reason, people think “should of” is the correct phrase.
In many English-speaking countries, English grammar is not a subject that people study in school so it is only to be expected that many people do not know this. I never studied English grammar in school and only studied English literature in English class.
When to use should have
We use should have when we talk about regrets.
Sorry, I am late, I should have left earlier.
Sorry, I am late, I should of left earlier.
This bus is taking ages! I should have taken the train.
This bus is taking ages! I should of taken the train.
Could of/Have, Would of/Have
“Could of” and “Would of” also don’t exist and are grammatically incorrect. The mistake is the same as above where the “‘ve” sound is misunderstood.
I could have been a contender.
I could of been a contender.
I would have gone out but I was busy.
I would of gone out but I was busy.
We use “could have” and “would have” to show alternative hypothetical situations in the past.
“Could have” shows alternative possibilities
“Would have” explains why alternative past situations took place.
Shudda Wudda Cudda
“Shudda” is a very informal version of “should have”. The reason people say “shudda” is because when people are speaking quickly and the “ve” contraction sounds like an “a”.
This also happens with “would have” and “could have” and there have been a few hit songs with these lyrics.