Quote Originally Posted by klopp View Post
I up this thread because I have some doubt about genitive as well.

I was studying my grammar book and there are some sentences construction about genitive I do not understand.

1) Я был в гостях у друга --------- what is for this у + genitive without a negation or the verb ectb? it is not possession as well... what does it mean?
When there is a preposition before a noun, it is always the preposition which governs the noun case. So, "у" always requires genitive.
Prepositions should not be translated directly from one language to another one. But there can be some hints to understand them.
The preposition "у" is more or less like English "at": Стол стоит у стены. - The table is (stands) at the wall. "Стена" is in genitive because of "у": у стены.

We often use this preposition to express possession as well: У меня есть книга. (At me there is a book. = I have a book). У моей сестры есть сумка. (At my sister there is a bag. = My sister has a bag).

The same preposition is also used to convey the idea "at someone's place" (although the word "place" is dummy and it is not used in Russian translation): Я был у друга. - I was at my friend. (= At my friend's place). У нас не курят. - At us (= at our place) (they) do not smoke. = Please do not smoke at our place. Какая у вас погода? - What is the weather at you? (= at your place, in your country, in your city etc.).

Quote Originally Posted by klopp View Post
2)I have to translate some sentences frome Italian, translated are (I read the transaltions in the book solutions) << Вчера был день рождения Тани, сестры Олега. В гостях у Тани были друзья Тани и Олега >>

Again I don't understand the sense of that у + genitive (is it something like "in her place?!" ) and that ending, сестры : why genitive? Oleg must be genitive no doubt about it, but Tanja shouldn't be nominative?!
"The sister" I think must be nominative, so--- сестра , and Oleg genitive. What am I misleading?
1. Please notice: день рождения.
2. В гостях у Тани is literally "in guests at Tanya". As I explained it above, yes, it is "at her home". "В гостях" is a fixed expression "when being someone's guest".

3. Вчера был день рождения Тани, сестры Олега. = Yesterday was birthday (lit. Day of Birth) of Tanya, of sister of Oleg. Here "сестра" is used to clarify who Tanya is, that is why "сестра" repeats the same case as "Таня": the birthday of Tanya (i.e. of Oleg's sister).

And "сестра" just cannot be in nominative as you suggest, since it is not a subject of the sentence

Quote Originally Posted by klopp View Post
3) other sentence: << на столе лежат книги Марио >>

why that ending for Mario, like the nominative? On the book ther is the same sentence for the person name << Марко >>
Foreign names ending in -о, -е, -и, -у, -ю, -э cannot be declined. Theoretically, in "книги Марио", the name "Марио" IS in genitive. But all the cases of this name just coincide: Марио, Марио, Марио, Марио, Марио, Марио.

Quote Originally Posted by klopp View Post
p.s. << в гостях >> What construction is? I still don't have studied that ending... just out of curiosity
As I mentioned it above, it is a set expression. Actually, there are two forms: "в гостях" (location) and "в гости" (direction).

"В гостях" (literally, "in guests") = not at home, at someone's place, being a guest there. A person at who you are staying (i.e. the host) is specified with the preposition "у". Or, it can be used without specifying a host: Не забывай, что ты в гостях! = Do not forget you are in guests (not at home)! It is a common saying to remind someone that a guest should respect the host's rules.

"В гости" (literally, "to the guests") means direction. The person to who you go is specified using "к" (+ Dative): Я иду в гости к другу. = I am going to guests to my friend (i.e. I am going to see my friend, to his place). It can also be used without specifying the host: "Мы идём в гости" = We are going to pay visit (to someone).