There is/are going to be twenty students.

twenty students 是负数那么当然用are

如果说 a crowd 的话
那么才说 There is going to be a crowd.
crowd是统称嘛

同样如果说 a class of twenty students
那么才是 There is going to be a class of twenty students.

若没有"a class of" 只是twenty students 的话
那么当然用 There ARE going to be twenty students.

用are.
There are going to be twenty students.
这其实是个倒装句，只是用的太多了，大家就习以为常了，真正的主语是be后面的名词，正常的语序是：
Twenty students are going to be there.但是没人会这么讲。

is..
因为要表现twenty students的be 动词是going to be中的be,但是这个be要用原形,我想为什么就不用我说了吧..而前面的be是根据be going to来决定的..

There will be twenty students here.

用are，此句是there are twenty students的将来时态。而且实际的主语是20 students。

呵呵 当然用are了 可以分析一下这个句子 一般现在时是There are twenty students 换成将来时就是There are going to be twenty students 谓语动词不变