Quote Originally Posted by renataf View Post
Thanks, but I didn´t understand. This phrase is in accusative case, OK, but I just can´t understand why instead of prepositional case I have to use accusative case if in the phrase I used на(preposition). I saw after prepositions I use prepositional case, so семьея in prepositional case is семьеe, right?.
Sorry, but I´m really confused.
I think your confusion comes from the designation "prepositional case". This is not intended to mean that it is the case which is always used after any preposition. Instead it is a case which is only used after certain prepositions, and never on its own. That's why it is called prepositional case. All the other cases except the nominative are also frequently, but not exclusively, used after certain prepositions, and each preposition is connected to one or more cases. I get the impression that most Russian prepositions are connected with the genitive case, but they are used more rarely than prepositions connected with other cases. Depending on the meaning to be conveyed, a preposition can also be connected with different cases. For example, на + accusative is used for movement towards a place, whereas на + prepositive is used for being at a place. So на работу = towards work, на работе = at work.