为什么句中要用助动词DO 而不用ARE?

discard2

Never before in the history of music have musical superstars been able to command (such extrordinary fees as they do today).

为什么句中要用助动词DO 而不用ARE?

Freedman's survey showed that people living in small towns and rural areas consider themselves (no happier than do people living )in big cities.
这句话中为什么又要用助动词DO 而不用are?

Building large new hospitals in the bistate area would constitute a wasteful use of resources ,(if only because the duplication of facilities should be avoided)
why not choose"solely in that the duplication of facilities should be avoided)

thank you

primefang

1  用are会有歧义， 是指 are able to 呢？ 还是 “是”？ 用了" do" 就只能指command

2  do 指consider, 非常正确阿

3  请参阅princess 的贴子，我们讨论了很多