What is the difference between some and an indefinite article

Question

Is there any difference in meaning between the sentences in the following pairs?:

1. • There is a person in the room.
   • There is some person/someone in the room.
2. • What would you do if someone/some person tried to rob you?
   • What would you do if a person tried to rob you?

I know that “some” is used in different senses, one of which is that it is used to refer to a person or thing that is not known or identified. I also know that “a(n)” does not necessarily mean one in all the sentences where it is used, and that according to English grammar an indefinite article is used also to refer to a person or thing not already known to the speaker, listener, writer and/or reader.

I think I've read somewhere in an English grammar book that “some” is also an indefinite article. It can also be used with singular countable nouns (as in “there is some person in the room”), which implies that the identity of the person is unknown to the speaker. On the other hand, this is not necessarily the case when “a” or “an” is used. “There is a person in the room” implies that the identity of the person may be known to the speaker and unknown to the listener or unknown to both or either the speaker and/or the listener. I want someone to shed some light on this.

Answer 1

It's not an article, but it's a determiner. At least, that's how Collins lists it.

Also from Collins, a determiner is:

a word, such as a number, article, possessive adjective, etc, that determines (limits) the meaning of a noun phrase, e.g. their in "their black cat"

Informally, one could say a determiner is like a article, only more powerful.

EDIT: As for the difference in meaning, there is none. The difference is pretty much only semantic. If I saw someone prowling around my neighbor's yard, I could say:

There's a robber at the neighbor's house!

or:

There's some robber at the neighbor's house!

and the meaning is the same: an unknown robber as the house next door.

When the noun that follows is singular, some and a mean the same thing. But some is flexible enough that it can be used with the plural, too:

There are some robbers at the neighbor's house!

We couldn't use a in place of some in that sentence; however, we could omit the determiner altogether:

There are robbers at the neighbor's house!

or we could use a different determiner:

There are three robbers at the neighbor's house!