Before a noun with a determiner (articles, demonstratives and possessive pronouns), we use some of. If there is no determiner we use some.
- Some of my friends live abroad. (NOT Some my friends live abroad.)
- I have bought some apples. (NOT I have bought some of apples.)
- Some people want a new system. (NOT Some of people want a new system.)
Some of is also used before a pronoun.
- Some of us want a new system. (NOT Some us want a new system.)
Nouns can be dropped after some, if the meaning is clear.
- I have bought too many chocolates. You can have some. (OR You can have some of them.)
Some with singular countable noun
With a singular countable noun, some can refer to an unknown person or thing.
- Some idiot has let the cat in.
- There must be some job for me.
- She is living in some city in Japan.
Some can mean ‘quite a’ in informal American English.
- It was some evening! (= It was quite an evening.)
Some can also be used to suggest that we do not think much of somebody or something.
- I don’t want to waste the rest of my life doing some boring jobs.
- She is going to marry some farmer in Kenya.
Some can be used with a number to suggest that the number is an impressive one.
- We have already sold some twenty thousand copies of this book.