'do' and 'copula' confounding?

[Mark Mitchell]

Greetings all.   I am writing a thesis in applied psycho-ling and I was
wondering if some list-members might be able to answer some nagging
questions.  Does one ever see a confounding of 'do' and copula in English
L1 acquisition  ( e.g. "It don't big" or "It isn't run" ?  These are
exceedingly common errors in English L2 acquisition).  Pinker refers to
the "meaninglessness of 'do'" as an explanation for over-tensing errors
(e.g. "Does it goes?") but also postulates that children may learn to
associate 'do' with infinitival verbs.  Thus, 'do' may have a kind of
implicit meaning for the child, as a verb marker (there is some evidence
that 'ing' sometimes plays a similar role, as a verb marker, in English L2
acquisition.)

The underlying central question here is: "Do children ever really utter
something (productively, generatively) that does not yet carry some
meaning (however implicit) for them?

I am aware of research showing very early stage children can already
perceive  the semantic difference between indefinite and definite
determiners.  And philosophers from Aquinas to Kant have written about the
meaning of the copula (which I have failed to really understand) but
Langacker, in his cognitive grammar presents a more accessible gloss,
wherein the addition of a copula can profile the extension of some
relationship through a span of conceived time.  Thus, "the cat on the
table" differs from "the cat IS on the table" in its semantics.  May not
even a fairly early stage child perceive such a semantic distinction?  If
both copula and 'do' already have some kind of meaning for the child,
prior to their productive use, then I would not expect to see confounding
errors.

Much obliged to all in advance.

mark mitchell