The textbook "The Cambridge Grammar of the English Language", page 67:
There is one guard outside.
It’s clear that it’s a forgery.
In the existential example, the dummy (semantically empty) pronoun "there" is subject, while"one guard" is displaced subject, and similarly in the extraposition example the dummypronoun "it" is subject and "that it’s a forgery" is extraposed subject.
Why do the noun phrases in bold not have any articles?
I think "subject" is a countable noun and, therefore, must have an article:
— if it were "a", it would mean "an example of a";
— if it were "the", that would mean such a type of subject is the only one in the sentence.
So I think using both "a" and "the" would be correct.
P.S.: As you see, the title example { "He" in "He left" is subject. } is different from those inside the post. I did so to make the title more clear and readable. I hope that will not affect anything.