Thus, I would have wanted to know your opinions first, and also would be advised about which theory is currently predominant in American English.
Hi, Deepcosmos—I do not wish to pass judgment John Lawler's theory, in part because I don't have time to study it and have little desire to. What I will do give you my own understanding of the syntax, as I learned it from mainstream generative syntacticians in the U.S. (of the highest caliber the world over).
1. He seems to be happy.
2. It seems that he is happy. (formal style)
3. It seems like (as if) he is happy. (informal style)
In sentence 2 and 3, I assume that;
1) the conjunctions - that and like, as if in informal style - lead not a subjective complement but an extra-posed subject clause (that is, preparatory it + complete intransitive verb - seem + extra-posed subject clause).
2) seems is justified to function as a complete intransitive verb, which leads an extra-posed subject clause.
I disagree with both your assumptions and will concentrate my discussion on the syntactic distinction between (1) and (2). First, in mainstream generative grammar, seem is a verb that only takes one argument in underlying structure.
The underlying structure of "He seems to be happy" is "__seems for him to be happy," and the underlying structure of "It seems that he is happy" is "__seems that he is happy."
As you can see, in each case, there is no subject in underlying structure. But English is a language in which clauses are required to have subjects. So something must occur in each case to supply the clauses with subjects.
What happens in "__seems for him to be happy" is that the complementizer "for" deletes and the singular masculine personal pronoun "raises" from the subject of "be happy" to become the subject of "seems" in surface structure.
What happens in "__seems that he is happy" is that dummy "it" gets inserted in surface structure. Nothing is extraposed. "That he is happy seems" is ungrammatical. The insertion of it has been called "It-Intraposition."
Of course, we also have sentences like "He seems happy." In most grammar books, and for almost every ESL teacher, "seems" is just a linking verb there. In advanced grammar, however, an embedded "small clause" is involved.