肉体派对这部电影谁有:Who is coming here is unknown?这句话又问题吗？

"who is coming here"整个是一个主语从句，后一个"is"是整句的系动词。意思是说，谁来还不知道。

Marry is late for school again,as is often.后半句对吗？
后半句正确，省略了同主句主语相同的成分，完整的句子应该是：Marry is late for school again,as she is often.后半句中的"is"也可省略。

I have been doing my excises since early morning.为什么要用been?
这是个现在完成进行时的句子，表明从过去某点起到现在一直在进行的动作。形式是：have/has been doing sth.

Were I your mother,I should advice you not to there.是不是应该not to go there 或者是not go to there？
如果按照语法的话确实应该加个动词，但是如果把"there"活用成名词的话此句也可以说。
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补充回答：
(have/has)been表示主动还是被动要看后面的动词形式。在你这个问题当中是表主动的。举一个被的来说：
The reports have been updated just now.报道刚刚更新。
至于独立主格的相关知识，请点击参阅：
http://zhidao.baidu.com/question/8152970.html

顺便纠正一下musicomet 同学的错误，"楼上的,第二句应该是"as she often does"吧 ",此句是一个典型的主系表结构She+is+late for school.所以后面仍需用系动词is，而不能用does.

有问题！！

Marry is late for school again,WHICH is often.

I have been doing my excises since early morning.用been doing表示“一直在做”
Were I your mother,I should advice you not to go there.

who coming here is unknown?

marry is late for school again,as she often do
因为since
were i your mother,i should advise you not to go there

alps`````````````厉害~佩服~~
支持

楼上的,第二句应该是"as she often does"吧