Okkk, we all know that "some" is usually used in affirmative senteces and "any" in negative or interrogative sentences.
"Why don't you buy some books?"
That's a negative + interrogative sentence. HOWEVER, we didn't use "any". I know that "any" in this sentence would totally change the meaning of it, but how can I explain to a student that grammatically this is correct??? Why shouldn't we use "any" instead of "some"?
To use "any," the question would have to be "Why didn't you buy any books?" This is a legitimate asking for information, and the word "any" suggests "even one."