Questions about the grammar of than were
energetically debated in C18, and still today are
sometimes asked. By origin it is a subordinating
conjunction, used to introduce comparative clauses,
He knows more than I do about the family history.
The use of the subject pronoun I anticipates the verb
(do), and confirms that a clause is to follow. This is
proof that than is indeed a conjunction – in that
sentence. But older commentators were inclined to
think, “once a conjunction, always a conjunction,” and
to disregard common constructions like the following:
He knows more than me about the family history.
In that alternative version of the sentence, the object
pronoun me shows than operating as a preposition,
which normally takes an object. Prepositional use of
than with an object pronoun has been recorded since
C16, yet prescriptive grammarians still argue that the
subject pronoun is the proper one to use after it; and
they would “correct” the second sentence to:
He knows more than I about the family history.
To many people this sounds less natural, but its
proponents argue that it is an elliptical version of the
first sentence above, i.e. that a whole clause is to be
understood after than, and so I is the correct pronoun.
Yet there’s no need for this elaborate argument if we
allow that than is both a preposition and a
subordinator. Research associated with the Longman
Grammar (1999) showed that speakers mostly use
than (and as) as prepositions (i.e. with a following
object pronoun) and only rarely with a following
subject pronoun. Fiction writers make about equal
use of the two constructions, while academic writers
use neither. Academic comparisons more often turn
on correlative phrases with comparative adjectives:
He possessed a greater sense of history than others
of his time.
You could therefore say that the problem is academic!
It rarely comes up in academic prose, and in fiction
and conversation where than is much more often used
with simple pronouns, the use of object forms is quite
idiomatic. In practice the issue only arises with first or
third person pronouns that have distinct forms for the
subject and object (I, we, he, she, they). For the second
person pronoun you, the third person it, it makes no
difference – or for nouns and proper names: He knows
more than John (does) about the family history.
A different issue with than is its potential
ambiguity when used elliptically, as in:
She’s kinder to her dog than the children.
To settle the ambiguity in sentences like that, the
point needs to be spelled out more fully. (See further
Combinations with than:
1 Than and what. The most extended use of than as
a preposition is to be seen in nonstandard usage such
as: He wanted it more than what I did.
Such constructions provide an empty object for
than but ensure the use of the subject pronoun in the
following clause. It could thus be seen as a kind of
hypercorrective response to the grammatical
“problem” (see hypercorrection). The what is
unnecessary because the sentence could perfectly well
be: He wanted it more than I did (or more than me). The
construction than what is associated with impromptu
talk – one of the various redundancies that occur
when we construct sentences on the run, which need
to be edited out of written documents.
2 Following than it’s possible to use either an
infinitive or an -ing form of the verb. Compare:
She rushed on rather than let us catch up.
She was rushing on rather than letting us catch
But as these examples show, the choice can effectively
be made by matching the forms before and after than:
the -ing follows a continuous/progressive form of the
main verb, while the infinitive goes with other aspects
3 Than with quasi-comparatives. A number of
adjectives and adverbs imply comparisons without
having the standard comparative suffixes such as -er
or more. They include collocations like
different/differently than (and superior than) which
are used especially in speech as alternatives to
constructions with from or to. Other constructions
which sometimes use than are sequences such as
hardly . . . than, scarcely . . . than, where the
alternative is to use when as the subordinator. Purists
are inclined to argue that than has no place in such
phrases, because the comparison remains implicit
rather than explicit in the form of words. Yet common
idiom endorses such combinations. See further under
different from, hardly and scarcely.
Taken from: http://www.cambridge.org/uk/linguistics/peters/