GRAMMAR LADY

BBRRRIIINNNGGGG... the phone rings just as I am on my way to a meeting .
"Hi! Can I ask a question about grammar?" asked the teacher. "There is a cat on the sofa. Is 'there' a subject? What is the grammatical term for there?"
I hang on the the receiver and indicate to the people waiting for me that I will be ready for our meeting in a minute.

"Well... In this instance, 'there' is empty, gramatically speaking. It is NOT the subject. It is used in this case because in English, we usually don't start sentences with indefinite subjects," I begin.
"But how do I explain this to my students?" she whined. "There must be a rule and a grammatical term for 'there'! And what if a student asks me, 'What is on the sofa?' Shouldn't the answer be, 'A cat is on the sofa.' Then the sentence would start with an indefinate subject..."

KILL ME NOW! I have been called upon to function as a dictionary, a tape recorder and lots of other such things in Asia, but never GRAMMAR RULE FABRICATOR! This should definately be added to the list of jobs which Taiwan is so in need of that there is a visa-processing fast track. I would insist my company hire someone this instant if the candidate could make up convincing rules and workable grammatical terms to satisfy.

As it turns out, there is an answer to that teacher: "There" functions syntactically as a grammtical subject. However, being the existential "there" (as opposed to the locative adverbial "there"), the subject is "notional" (or empty). I wonder if she's prefer this explanation for her 13 year old students? I found this in the Longman Grammar of spoken and written English (the online Grammar Lady unfortunately passed away recently).