Hi Mesmark,
I like your question.
I keep my copy of
Oxford Pocket Basic English Usage by Michael Swan very handy when I'm teaching classes that enjoy persecuting me with difficult grammar and language questions. To my most persistent class and to the language-curious, I (politely) suggest the students buy a copy for themselves. Most questions students ask can be found even in the pocket version. During my sales pitch would be the only time my students see the book in class, though, because I don't recommend rustling through the pages during class time. (No joke, a doctor once did that while I was in his office and it didn't inspire my confidence, to say the least.)
I try to anticipate which questions will come up in class and prepare for those, but there seems to always be one student who asks random things that are completely off topic. Then I do much like you do and try to think of more examples and give an instinctive answer. I check the
Usage book during break time and tell them what I've found afterwards.
To your question above:
"There is a man at the door.
vs. A man is at the door."
I would say there isn't much difference in meaning, but the first sentence is more common.
According to Michael Swan
(I like to refer to him like he's my closest friend, and in grammar classes he is
), we use the expression "There is/are..." when we tell that something
exists
compare: There's a hole in my sock. (OK) A hole is in my sock. (not OK).
I'm not sure how to explain why "A man is at the door" does sound OK.
The structure is used with "indefinite subjects" -nouns preceded by
a/an, some, any, no, or no article, or with
somebody, anything, nothing and can be used with any simple tense of the verb
be, or present and past progressive forms of other verbs.
So maybe "A man is at the door" is less indefinite, perharps it's implied someone must know who the man is.
As for the question "Yeah, but where did this "There is.. " structure come from,
why do you say it that way?
Why, teacher?
Why,
why?" I consistently fall into the "How can anyone know?" argument, which annoys some students. Sometimes I throw back a why question from their own language, like "Why do Koreans use Korean
and Chinese numbers when telling the time?"
In the end, I consider getting myself a Ph.D in linguistics but then shortly afterwards I get distracted colouring flashcards for a kindergarten lesson and put off that plan.
So what do you think, are language teachers supposed to answer the Why? questions as well as explaining the hows?