>>: Anticipatory "it" can also be a subject:

>>: It seems to me that your question is an interesting one.
    ^^             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Note that, if it mean anything in this sentence, it means exactly the same thing as the that-clause at the end.
>>: Me parece [no "it"] que tu pregunta...
Note here that it used to be possible in English to say
Meseems your question is an interesting one.
which has exactly the same structure as the Spanish. But we've lost this construction.
>>: It is necessary that every man do his duty.
>>: Es necesario [again, no "it"] que cada hombre...
Here (though not with seem, which has a lot of peculiar syntax), there's another construction still possible (though quite awkward and possibly becoming extinct), in which the it isn't used and the that-complement retains the subject role:
That every man do his duty is necessary.
This particular it is called anticipatory because it does anticipate (= refer to) a complement clause (often a that-clause, but infinitives also occur:
   It's not so easy to fool little girls as it used to be.
   ^^               ^^^^^^^^^^^^^^^^^^^^
It is indeed the subject here, as witness its retention in the equative clause; but since it's equivalent to the complement, the complement is also in some sense "the subject". Complement, in fact, means "clause functioning as a noun", normally either subject or direct object, occasionally as prepositional object or other oblique noun.

The whole thing goes under the name of Extraposition in the trade, and seems to work to keep subject complement clauses from assuming the normal subject position preceding the verb. The usual explanation for it is that English, being a right-branching language, and having a decided preference for getting at least an auxiliary verb, if not the whole verb, to be the second word in the sentence, dislikes having "heavy" elements like clauses usurping the beginning of the sentence. So the clause is demoted and sent to the end of the line and a usurper ('dummy') it takes the place, allowing us to get to the verb right away and figure out what the subject is in our own good time. Seems to work OK.

In an inflected language like Spanish, there's sufficient freedom in word order to allow subjects to be placed just about anywhere on a stylistic whim; but English is rather fussier about where things go, and it just won't do to have subjects lying about anywhere at all. You have to have something up there at the beginning, and it is the traditional choice.

Other 'dummy it' constructions include:

  It's a long way to Tipperary.    [distance it]
  It's raining, it's pouring.      [weather it]
The test for extraposition it is whether there's a complement clause (tensed that, infinitive, or rather less frequently a gerund) at the end of the sentence, and whether one can replace the it with the complement clause and have it make sense.

I say make sense because sometimes extraposition is obligatory, as it is with seem. The sentence

*That he's an idiot seems to me.
is of course ungrammatical, though the extraposed version
It seems to me that he's an idiot.
is fine.

--- Followup --

> But note that both

>     That he's an idiot seems clear to me.
> and
>     It seems clear to me that he's an idiot.

> are grammatical.  Is it only with "naked `seem'" that
> extraposition is obligatory?  Seems true to me.
I really should have known better than to open that particular can of worms in a.u.e; I said that seem had some pretty crazy syntax, remember.

This is a bit hard to explain in ASCII; bear with me, please.

First, some equivalences:

>     That he's an idiot seems clear to me.
   =  That he's an idiot seems to be clear to me.
   =  That he's an idiot seems to me to be clear.
   =  It seems to be clear to me that he's an idiot.
   =  It seems to me to be clear that he's an idiot.
In other words, clear after seem really is the remains of an infinitive with to be; this is pretty common with adjectival predicates. More crucially for this example, both clear and seem are experiential predicates, and refer to an experiencer (marked with to if present), and both of them take complements.

Notice, in fact, that the grammaticality of the sentence depends on the fact that clear takes a complement:

[That he's an idiot] is clear to me.
and the grammaticality of the extraposed version depends on the fact that clear allows extraposition:
It's clear to me [that he's an idiot].
This can be seen from the fact that a predicate that doesn't take a complement -- say, red -- will not allow a complement nor extraposition with seem:
The wall is red.
*That he's an idiot is red.
*It seems red (to me) that he's an idiot.
The wall seems (to be) red (to me).
*That he's an idiot seems (to be) red (to me).
*It seems red (to me) that he's an idiot.
Even things that ought to mean something seem (to be) forbidden by the syntax of seem, since extraposition is obligatory here.
It seems (to me) that the wall is red.
*That the wall is red seems (to me).
Now, as to 'naked seem' -- that's a cute phrase, but it might be taken to indicate that seem certain is a compound verb, which isn't really the case; it's a derived phrase resulting from deletion of an infinitive to be in a complement. Clause union is the technical term. What's going on in this case is another complement phenomenon that I was hoping to avoid having to mention, since it's complicated and is easily confused with some other things, but, since you asked ... The name of this rule in the trade is Raising or Subject-Raising, and the best source is a book by Paul Postal called On Raising from the early 70's. Raising means separating the subject of a complement from its verb phrase and making it the subject of the main verb.
This can only happen with certain main verbs; and seem is one. Examples:
  *[The wall is red] seems (to me).                    Original
   It seems (to me) [that the wall is red]             Extraposition
   The wall seems (to be) red (to me).                 Raising
   ^^^^^^^^       (^^^^^) ^^^
   [That Frank is the culprit] is widely believed.     Original
   It is widely believed [that Frank is the culprit].  Extraposition
   Frank is widely believed [to be the culprit].       Raising
   ^^^^^                     ^^^^^^^^^^^^^^^^^
   [That the payroll will be stolen] is likely.        Original
   It is likely [that the payroll will be stolen].     Extraposition
   The payroll is likely [to be stolen].               Raising
   ^^^^^^^^^^^            ^^^^^^^^^^^^  
Obviously, it's not Frank who's widely believed, nor the payroll itself which is likely; both of these are transforms of the original sentence. A somewhat hairier version of the constraint I cited in my original post (that Extraposition was obligatory with seem) would be that either Extraposition (of a that-clause) or Raising (from an infinitive) was obligatory with seem:
   *[That communism is dead] seems (to me).     Original
    It seems (to me) [that communism is dead].  Extraposition [that]
    Communism seems (to me) [to be dead].       Raising  [infinitive]
    Communism seems dead (to me).               Raising + to be-deletion

Thus, in the sentences adduced:
>     That he's an idiot seems clear to me
> and
>     It seems clear to me that he's an idiot.
the derivation goes:
 *[[That he's an idiot] to be clear] seems to me.   Original
  [That he's an idiot] seems [to be clear] to me .  Subject-Raising
   ^^^^^^^^^^^^^^^^^^         ^^^^^^^^^^^
  [That he's an idiot] seems clear to me.           To be-Deletion
  It seems clear to me [that he's an idiot]         Extraposition
There's quite a lot more that could be said about these, and about allied phenomena, but I hereby forbear. Remember, you asked.
  - John Lawler       Linguistics Department and Residential College     University of Michigan

    "Language is the most  massive  and  inclusive  art  we know,  a           - Edward Sapir
      mountainous and anonymous work of unconscious generations."       Language (1921)

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