There was someone on Facebook who commented like this:
Next time she should of flushed the toilet.
At first, I thought he had misspelled "should have", then I was thinking that it's not correct to use should have (right?) in the future construction (I suspect 'next time' he meant to say about future). I also checked the Ngram viewer about the usage of should of and should + of + past participle (I randomly chose known).
These are the results, by the way:
The usage of should of
The usage of should of known
Now, my question is, does that construction exist? (I've never seen it before) If that's just a misspelling, then why is the construction used in many books (According to Ngram viewer)? What construction is it called then?
Best Answer
Why does that construction exist?
Because in spoken language, should have and should of are indistinguishable (when enunciated as contracted should've). And some native speakers are so ignorant they don't understand that basic syntax requires an auxiliary verb after should, not a preposition.
But note that some perfectly competent Anglophones may deliberately write the incorrect version sometimes as a facetious usage. OP's cited Facebook poster probably isn't very competent, given that "next time" is future, so the auxiliary have (part of a "Present Perfect = Past" verb form) shouldn't be there anyway. But even people who know perfectly well that it's "incorrect" sometimes write things like a whole 'nother ballgame for much the same reason.
I don't know why OP wrote it's not correct to use should have. That's exactly what the Facebook writer should of1 written!
1 In case it's not obvious, that's me being facetious! But actually whereas one should've = should have done something in the past, when talking about a future "next time" it's what one should do (infinitive do, not have + Past Participle done).