One of the trickiest commas is the comma in between two or more adjectives that modify a noun. Is my hot chocolate a warming at-home beverage, or is it a warming, at-home beverage? This could be a matter of opinion, but I think most people might leave the comma out, which would make the two adjectives noncoordinating, i.e., one is subordinate to the other.
However, my good friend from Barcelona, Spain, likes to have a small, rich caffe latte to feel at home. The drink should come in a small cup and the coffee should be rich, in comparison to the typical American cup of coffee that is. The two adjectives, small and rich, are coordinating, or equal in status. Use a comma.
It’s clearer with some pairs of adjectives: I enjoyed a refreshing cucumber drink at a French restaurant in Guanajuato, Mexico. No comma.
How do you know when to use a comma or when not to? If the adjectives coordinate, then put the comma in. If not, leave the comma out. The Purdue Online Writing Lab gives some clarity (see item 6).