In my university, in English Grammar, I learned I can use "any" for negative and interrogative sentences and "some" in positive sentences, but when I watch English movies, "any" is used in positive sentences. So does this grammar rule have any utility?
As you've stated it, the rule is completely wrong. However I think they are referring to this rule concerning the words some and any:
- In the positive you normally say: "You took some cookies."
- In the negative you normally say: "You didn't take any cookies."
- In the interrogative you normally say" "Did you take any cookies?"
However there are lots of variants on this: "If you took any cookies..." is normal, and some people would say: "Did you take some cookies?", especially if it was "Did you take some cookies out of the tin?"
It's a decent guideline for getting the common usage, but it's by no means a hard and fast grammar rule.
Both some and any have plenty of other uses, and you shouldn't generalise to them.
Simply put.. (Keeping with the cookie theme.) You are somewhere with a few dozen cookies.
Someone says: "Have some cookies" - in this case, "some" means "not all". You may have some cookies, but not all. If you want two, then that is ok.
Someone says: "Take any cookie" likely means to take "one cookie"
"any" might mean "one" or "somewhere between 0 and all".
Other examples. At the car rental place, "I'll take any car." I've never heard "I'll take Some car".
"I need a volunteer. Any one will do." Therefore, I need one person. "I need some volunteers. Any one of you." I need more than one person and there is no qualification needed."
Most of the time, "some" and "any" are similar but not interchangeable. However, English is very forgiving in communicating understanding, even if it is difficult to get "right".