Normally, copulas hold a subject complement (or a predicate in any case). Example.
The sky became clear. I am ill.
But what is in the definition of a lexical verb that makes copulas lexical verbs?
As the comments point out, I ought to be more specific.
I have recently read an article of French linguist Pollock.
Pollock, Jean-Yves. 1989. Verb movement, UG, and the structure of IP. Linguistic Inquiry 20, 365-424.
He claims that the differences between English and French with respect to syntax of sentence negation, questions, adverbs, floating quantifiers and “quantification at a distance” are correlated. He proves this in various ways. One thing that bothers me, though, is that he seems to contrast copulae and auxiliaries, but in such a strange way I have never heard of before.
Consider the following examples (from Pollock, aforementioned).
1. être (to be) and avoir (to have)
- Ne pas être heureux est une condition pour écrire des romans.
- Ne to not be happy is a prerequisite for writing novels.
- N'être pas heureux est une condition pour écrire des romans.
- 'Ne to be not happy ...'
As you can see, the negation in French can be in front or after the verb (a copula). More correctly, it is the verb that moves to Infl - in front of the negation - or not. Verb Movement is possible, but not obligatory in this case.
Then Pollock continues by saying "Let us now consider lexical verbs. The situation here contrasts sharply with the paradigm [in the previous examples]."
2. Lexical verbs
- Ne pas sembler heureux est une condition pour écrire des romans.
- 'Ne not seem happy is a prerequisite for writing novels.'
- *Ne sembler pas heureux est une condition pour écrire des romans.
- 'Ne to seem not happy ...'
(* meaning "not considered standard language" or simply not possible)
This tendency in infinitives "sort-of" aligns with English tensed clauses.
- He is not happy.
- *He seems not happy.
- He was not arredsted.
- *He got not arrested.
"It appears that although Verb Movement can apply to auxiliaries and lexical avoir, it cannot apply to lexical verbs in infinitives in French."
Without going further into detail, it struck me that Pollock seems to make a distinction between to be (and French être) - which seems to be an auxiliary, according to his theory - and to seem (and French sembler) as a lexical verb. Even though, my gut tells me that être in
1. and in He is not happy is as good a copula as sembler or *He seems not happy..