Tip: See my list of the 
MostThe expression “would of” is a common mistake. People who learn English mainly by ear (including many native speakers, unfortunately) sometimes want to write what they hear as “would uhf” and assume it must be “would of” (which would be pronounced this way).
This does not make sense. What they want to say is “would’ve”, a contracted form of “would have”. For example, when someone writes “I never would of thought”, what he or she means is “I never would’ve thought” = “I never would have thought”. To summarize:
correct I would have done it.
correct I would’ve done it. (informal)
wrong I would of done it.
Similarly, “could of” is a misspelling of “could’ve” and “should of” is a misspelling of “should’ve”.
