How am I just now reading this?? I should OF read it years ago as a teacher…
Would of, could of, might of, must of | Sentence first:
Unstressed ’ve is phonetically identical (/əv/) to unstressed of: hence the widespread misspellings would of, could of, should of, must of, might of, may of, and ought to of. Negative forms also appear: shouldn’t of, mightn’t of, etc. This explanation – that misanalysis of the notorious schwa lies behind the error – has general support among linguists.