than

Từ điển American Heritage Dictionary 4th

than (thăn, thən)conj.
1. Used after a comparative adjective or adverb to introduce the second element or clause of an unequal comparison:
She is a better athlete than I.
2. Used to introduce the second element after certain words indicating difference:
He draws quite differently than she does.
3. When. Used especially after hardly and scarcely:
I had scarcely walked in the door than the commotion started.prep. Usage Problem
In comparison or contrast with:
could run faster than him; outclassed everyone other than her. [Middle English, from Old English thanne, than. See to- in Indo-European Roots.] 
Usage Note: Since the 18th century grammarians have insisted that than should be regarded as a conjunction in all its uses, so that a sentence such as Bill is taller than Tom should be construed as an elliptical version of the sentence Bill is taller than Tom is. According to this view, the case of a pronoun following than is determined by whether the pronoun serves as the subject or object of the verb that is “understood.” Thus, the standard rule requires Pat is taller than I (not me) on the assumption that this sentence is elliptical for Pat is taller than I am but allows The news surprised Pat more than me, since this sentence is taken as elliptical for The news surprised Pat more than it surprised me. However, than is quite commonly treated as a preposition when followed by an isolated noun phrase, and as such occurs with a pronoun in the objective case: John is taller than me. Though this usage is still widely regarded as incorrect, it is predominant in speech and has reputable literary precedent, appearing in the writing of such respected authors as Shakespeare, Johnson, Swift, Scott, and Faulkner. It is also consistent with the fact that than is clearly treated as a preposition in the than whom construction, as in a poet than whom (not than who) no one has a dearer place in the hearts of his countrymen. Still, the writer who risks a sentence like Mary is taller than him in formal writing must be prepared to defend the usage against objections of critics who are unlikely to be dissuaded from the conviction that the usage is incorrect.·Comparatives using as... as can be analyzed as parallel to those using than. Traditional grammarians insist that I am not as tall as he is the only correct form; in formal writing, one should adhere to this rule. However, one can cite both literary precedent and syntactic arguments in favor of analyzing the second as as a preposition (which would allow constructions such as I am not as tall as him). See Usage Note at as¹.