He is taller than she is. than引导比较状语从句，为正规用法。

He is taller than her. than被用作介词，后接宾格代词。这在早期被认为是错误。但犯同样错误的人很多，现在作为非正式用法已经被接受。词典也给出了than为介词的用法。但在国内考试时还是要尽量避免。

以下为《韦氏高阶词典》对than用作介词的解释：

: when compared to — used with pronouns in the objective case (me, her, him, them, and us) in the same way that the conjunction than is used with pronouns in the subjective case (I, she, he, they, and we)

• She is younger than me. [=she is younger than I (am)]

• I'm taller than him. [=I'm taller than he (is)]

Usage： Some people consider the use of than as a preposition to be incorrect. It is very common, however, especially in the phrase than me.