What you hear may not be what you should see...

WRONG: "He should of known better."

RIGHT: "He should have known better."
or
"He should've known better."

I got this on Facebook -- and I should know better than to pick on FB friends and their posts! But this was a teacher, and I guess I hold teachers up to a higher standard.

The problem comes from the sound. When the contraction form "should've" is used in speech, it does indeed sound as if the word "should" (or could, or would, or must) is followed by "of." So I'll try to refrain from picking on my FB peops, and hope that teachers will know better.

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A great place to go for help with all kinds of thorny writing problems and questions is Grammar and Usage for the Non-Expert, by Tina Blue. I find it to be a big help.