> Isn't the fundamental assumption here that kids learn to speak in
> grammatically correct sentences? Half of the college students I've taught
> at Research I and II universities - and more at the second tier small
> college I taught at, don't speak or write in grammatically correct
> sentences... is this a problem for the theory?
Nope. You're thinking of schoolmarm prescriptive grammar (don't say "ain't", don't say "axed" instead of "asked", etc.) rather than a linguist's descriptive sense of the term. What linguists mean about "grammar" is whatever it is that enables us to say that utterances like "The girl that Jack dated her dumped him" is ill-formed, and it's ill-formed because "her" doesn't belong there.
Not that sentences like this don't actually occur all the time -- that's part of the poverty-of-stimulus problem. It's that native speakers of English, regardless of dialect, will know that something is wrong -- that a mistake has been made, or that the speaker somehow changed horses in midstream (anacoluthon is the technical term).
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Michael Smith mjs at smithbowen.net http://stopmebeforeivoteagain.org