"Are they still here?"
I can not find a logical way to think ''are'' as a lexical verb.
Because "are" in this case is taken to mean "exist, be present". "Are" exists on its own, it is the only verb in the sentence, therefore it is impossible that it is auxiliary.
It might help to remember what an auxiliary verb is, if you remember what "auxiliary" inherently means:
Providing supplementary or additional help and support.
When something is an auxiliary verb, then it exists to assist another verb in the sentence.
Thomas is working.
"is" has no meaning by itself. The main focus of the sentence (in regards to verbs) is work. "Is" is only added to the sentence because it would otherwise sound silly.
"Is" supports the presence of "working", in my example.
Compare this to:
Thomas is here.
But in this example, there is no other verb, and therefore "is" cannot be auxiliary. It is lexical, because it stands on its own.
I've tried to explain it with common sense, rather than pointing to generalized grammatical descriptions that are often hard to understand. I hope my explanation helps you.
