What is the difference between:
- There is a person in the room.
- There is some person in the room.
“A person” implies a singular person. A single person is in the room.
“Some person” is the same thing but goes a step further. It implies that the one person is somewhat unknown. It could be a girlfriend, or a violent robber, or something in-between.
“Some” implies an unknown. (e.g. "Some amount of money.", or "Some outcome." etc.
That's how I see it. Hope it helps.