It seems like there is something that Russians never get tought, but ought too. this article, mostly deals with food, or items such as cereal, or gum, where you can't really divide it up into one's and two's, but piles of stuff. I'm talking about "some". I wil provide a few examples. where "an/a" kinda sound "funny", and it is better to use "some".

well, i first thought of this, when one of my russian friends, who speaks really good english said "Can i have a gum" and i luaghed... becuase i had never heard that before. It sounds much better to say "Can i have some gum" or "a piece of gum".

another example

Can i have some soda?
can i have some cerial
do you have some gas? my car is almost out!
do you have some water?

Like i said, this is mostly used with, food, or liquids, where you can't really ask for specific amounts, just "some" of something. usually, it can be interchanged with "any", and it just depends which one you want to use.

Do you have any water?
do you have any cerial?

but, to me, "any" sounds a little desperate.