That-clause in "it seems that"

You would never have a that-clause if "it" was not a dummy subject. (OALD, 3)

it seems | it would seem
used to suggest that something is true when you are not certain or when you want to be polite seem (that)… It would seem that we all agree. This last point, it seems to me, is particularly important.
adj. It seems only reasonable to ask students to buy a dictionary.

It seemed [that he was correct].

The subject "it" is semantically empty here, so it's not possible for the bracketed declarative content clause to function as PC, though it is an internal complement of "seemed".

The construction bears some resemblance to an extraposition construction, but differs in that the subordinate clause cannot occupy subject position: "*That he was correct seemed" is of course totally ungrammatical.

Thanks for the answers guys. While I was learning the new classification of finite clauses proposed in CGEL, I came across a paper by Huddleston and Pullum , on which a footnote says

6 In the construction with seem, which Seppänen & Herriman (2002) argue persuasively does not involve extraposition, the subordinate clause cannot occur in subject position: It seems that she’s right, but not *That she’s right seems. Here, then, there is no possibility of having a noun in place of the noun clause (without a change in the interpretation of it, i.e. a change of construction).

I have yet to read the work by Seppänen & Herriman but I believe I am not far from the answer. And I can see clearly my confusion arose because I mixed up the so-called nominal clause with a noun.