Lots of things annoy me about written English and this heinous crime is probably the most irritating of them all. In fact, it bothers me so much that I feel compelled to hit things whenever I see it.
In some regional accents, I totally understand that ‘should’ve’ may sound very much like ‘should of’, but just because it sounds like it, doesn’t make it right. Avoid leaving your mistakes to posterity and never, ever, write them down. The only time ‘of’ follows ‘should’ is in expressions that start with of and are separated by a parenthetical comma, e.g. I should, of course, eat less if I want to lose weight.
‘Should’ve’ is a contraction of ‘should have’ and should only be used in informal writing or reported speech. So if you find that you cannot simply contract the two words and insert an apostrophe to indicate the missing letters then don’t and instead write the two words in full. If, on the other hand, you feel an overwhelming urge to keep repeating your mistake, then I would suggest that you stand in front of a mirror taking a good look at the fool looking back whilst repeatedly hitting yourself over the head with a wet halibut.
There really is no excuse for lazy thinking.
Could of, Would of, Must of:
On a final note, it is just as infuriating to read ‘could of’, ‘would of’ and ‘must of’, so please keep me sane and just stop it. Stupidity doesn’t become charming in its repetition; just think of ‘Lost’.