Also, when talking about time, с + gen. means "from" that time, the oppostie of "до" + gen., "until":

Открыт с семи часов до двадцати трёх часов.
Open from 7:00 to 23:00.

and that c + inst. is used for the equivalent of phrases like "My friends and I":

Мы с друзьями (not "Я и друзья") смотрим телевизор.
We with friends (not "I and friends") are watching television.