The Auxiliary System in English is a major player in some important Transformations.
The complex auxiliary system plays a large role in many transformations, as you can see. So far, we have talked about two: the Dummy and the Negative Auxiliaries.
A third important group is the Aspect Auxiliary. When I say, "I go," I am declaring that to be something I do.
But suppose I want to convey the idea that I am in the process of doing it right now?
I say, "I am going." English structure does not allow for two consecutive verbs in a sentence. But here we apparently have the Copula "am" and the verb "going" back to back!
Shakespeare, of course, was not referring to grammar, but he might well have been, when he wrote, "...to be or NOT to be½that is the question!"
In this case, is "am" really the Copula or not?
NOTES: More on Auxiliaries.