Your question isn’t a small question at all. It is, in fact, one of the most difficult and complicated questions that have been posted in this Forum ever, and I’m afraid that I couldn’t possibly address all aspects of it satisfactorily here unless you send me the references that you say classify comparative clauses as a special type of clause.
The problem is in the nature itself of comparative clauses, for they are not at all times dependent or subordinate clauses. The most common comparative sentences are, in fact, ellipted forms that structurally speaking consist of only a single clause, as in the following sentences:
Ellipted, single-clause comparatives:
1. “You are taller than me.” (This is the ellipted form of the comparative sentence “You are taller than I am tall,” which consists of the main clause “You are taller” and subordinate clause “I am tall” linked by the comparative “than.” However, in the ellipted form, “You are taller than me,” the compound sentence has become a single-clause sentence where “than” functions as a preposition and “me” as the object of the preposition.)
2. “That guy is as annoying as she.” (This is the ellipted form of the comparative sentence “That guy is as annoying as she is annoying.” which consists of the main clause “That guy is (as) annoying” and subordinate clause “(as) she is annoying” linked by the comparative “as…as.” However, in the ellipted form, “That guy is as annoying as she,” the compound sentence has become a single-clause sentence where “as…as” functions as a comparative and “she” as the subject of the comparative.)
Comparatives as coordinators, not subordinators:
It’s also very common for comparatives to function as a coordinating conjunction rather than as a subordinating conjunction, as in the following sentences:
1. “More women than men visited our website last year.” (The comparative “more…than” in that ellipted sentence clearly functions as a coordinate conjunction, not as a subordinate conjunction because structurally, there’s evidently no subordinate clause to speak off. When unellipted, of course, that sentence takes this scrupulously grammatical but terribly repetitive and unwieldy form, “There were more women who visited our website last year than men who visited our website last year.” In that form, there are clearly two coordinate clauses linked by “than” as a coordinate conjunction.)
2. “Fewer serious mistakes were committed by the team this year than last year.” (The comparative “fewer…than” in that ellipted sentence clearly functions as a coordinate conjunction, not as a subordinate conjunction because structurally, there’s evidently no subordinate clause to speak off. When unellipted, of course, that sentence takes this scrupulously grammatical but terribly repetitive and unwieldy form, “There were fewer serious mistakes committed by the team this year than the serious mistakes committed by the team last year.” In that form, there are clearly two coordinate clauses linked by “than” as a coordinate conjunction.)
Comparatives as subordinators
In practice, there are decidedly fewer instances when a comparative actually functions as a subordinator in complex sentences, as in the following examples:
1. “More guests attended the wedding reception than we had issued invitations for.” (The comparative “than” is definitely a subordinating conjunction in this ellipted sentence, linking the main cause “more guests attended the wedding reception” and the dependent or subordinate clause “(the number of guests) we had issued invitations for.”)
2. “A better lawyer is representing them than we have.” (The comparative “than” is definitely a subordinating conjunction in this ellipted sentence, linking the main cause “a better lawyer is representing them” and the dependent or subordinate clause “(the lawyer) that we have.”)
I trust that the kinds of comparatives and the examples I presented above have sufficiently demonstrated that it’s probably not very advisable to classify comparatives as a fourth type of dependent clause. I think the most we can say is that comparative clauses are indeed a special type of clause, capable of being a single-clause comparative, a coordinate clause in a compound sentence, or a subordinate clause in a complex sentence.
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CORRECTION: In the 3rd and 4th paragraphs of the discussion above (Items 1 and 2), the term "subordinate clause" in the phrase "...and the subordinate clause 'I am tall'" and in the phrase "...and subordinate clause '(as) she is annoying," respectively, should read "coordinate clause" instead. This is in keeping with my preliminary appreciation of the sentences involved as compound sentences, which I argued become simple sentences when ellipted. My apologies for the wrong terms. (February 7, 2015)
