有个关于助动词的低级问题

很多结构要求助动词尽量补出，可是我一直没搞明白哪些属于助动词，还有它的补出形式

1、助动词是不是一般有do,be, have/has, will/shall这几种阿？

2、比如，比较结构中要补出do，前半句我们假设动词是pass, 那是补do，还是补pass？

同样，be动词，是补出be呢？还是根据主语补was/were？

很基础的问题，但是一直没弄懂，请帮个忙，谢谢

补个例子,大全763中

The guiding principles of the tax plan released by the Treasury Department could have even a greater significance for the economy than the particulars of the plan.

(A) even a greater significance for the economy than

(B) a significance that is even greater for the economy than

(C) even greater significance for the economy than have

(D) even greater significance for the economy than do（D）

(E) a significance even greater for the economy than have

C和D的区别是do和have，这道题里选do因为照应前面could have，那有没有have当助动词补出的时候阿？

一直有这么个疑问，望有大侠可以解答