The English apostrophe s and s apostrophe cause a lot of problems, even for native speakers. This lesson's task is to help you learn about possessives and contractions that need apostrophes and plurals that don't.
What's the difference between their, there, and they're? They're really not that complicated; once you understand their differences, there shouldn't be any more confusion.
The phrase could have refers to something that was possible but did not occur in the past. In informal speech, it is contracted to could've, not could of.