*Dons Grammar Nazi outfit*
My dearest ladies and gentlemen, Please use 'Could have', 'Should have' and 'Must have' instead of 'Could of', 'Should of' and 'Must of'.
If necessary, use the contracted form of have: "could've".
'Of' is a preposition. More correctly, it is an adposition. See http://en.wikipedia.org/wiki/Preposition
. If used instead of a verb, the created sentence is grammatically incorrect.
Indeed, when enunciated the two spellings are not homophones.
'Of' is pronounced /ɒv/ or /ʌv/, while 'Have' is pronounced /hæv/, /(h)əv/, or /hæf/.
Thank you for listening to this public service announcement.
*takes off Grammar Nazi outfit*
Dark_dragon now apologises profusely.