I’m (reasonably) sure these are wrong:
- I would of won.
- I could of done that.
and are likely so common because if you phonetically transcribe “would’ve”, “could’ve”, etc, that’s what you get. Nevertheless, I commonly make that mistake over and over again. How can I definitively correct this error?
Remove the modal verb (would, could, should, etc.) from the sentence, and see if it still makes sense.
- I would have won. -> I have won. (works!)
- *I would of won. -> I of won. (nope, that’s not right…)
You can be sure that if the sentence is supposed to have have, then you can always remove the modal.