例句16-18，为什么前两句as和than用作介词,后两句用作连词，连词和介词用法有什么区别

Question

如题所述

Answer 1

than做连词的时候，意思是比。比用于引入不相等比较的第二成份或句子：
She is a better athlete than I.
与我相比她是个更好的运动员
He is taller than his brother.
他比他兄弟高。

而作为介词的时候，是和。。。比较，后面不可以接句子，只能接词或短语。
如 disliked no one more than her.
没人比她更惹人厌

as 作为连词，意思是（用于比较）与…一样
He can run as fast as I can.
他能跑得和我一样快。
She works in the same building as my sister.
她和我的妹妹在同一大楼上班。
This is the same as it was before.
这个与以前一样。

而作为介词，意思是：
正如，照
as you know
正如你知道的
就…而言
He's quite good, as boys go.
就男孩儿而言，他相当不错了。

像…
（指处于某种状态、 性质、情况、工作等之中）
He works as a driver.
他以开汽车为业。
The kitten uses that box as a bed.
小猫把那盒子当作床。追问

那为什么上面两句判断不能使用连词，比如说she is younger than I am

或者she is as old as me,是什么东西判断应该用连词还是介词