关于不定式的三个问题1.The man we followed suddenly stopped and looked

关于不定式的三个问题
1.The man we followed suddenly stopped and looked as if to see whether he was going in the right direction.
2.The flu is believed to be caused by viruses that like to reproduce in the cells inside the human nose and throat.
3.Do let your mother know all the truth.She appears to have been everything.
请问：
1.请问as if引导的是不是状语或表语从句?因为如果是,这个从句的谓语动词应该用虚拟语气才对,但却用了不定式,请问是怎么回事?
2.为什么使用to be caused?答案写的“根据句意和介词可知是被引起”,根据介词能得知什么?
3.首先这个句首的do是什么作用?请问为什么使用to have been?