其实:
I'm smarter than Yuan Li. = I'm smarter than Yuan Li is.
I'm quieter than he is. = I'm quieter than him.
Maria's best friend is quieter than she is. = Maria's best friend is quieter than her.
一般来说,有谓语动词的是比较完整的句子,而没有谓语动词的是简略句子。
如果than后面是名词或人称代词的主格的话,这个是可由可无的。
I'm quieter than him.
Maria's best friend is quiter than her.
有 和没有都对