Nouns: Case: Genitive

Genitive case describes (primarily) possession or relationship, often in English expressed with 's or the preposition "of".  It answers the questions: кого? чего?  whom? whose? of what?

 It is also used for other things, mainly:
 1. The “accusative of time” is used, often without a preposition to express duration or frequency.

• Я здесь уже неделю = I've been here (for) a week.
• Раз в год = Once a year

2. The “accusative of motion” is used with в and на to express movement towards (instead of location), “into” and “onto”

• Мы едем в Москву. = We're going to Moscow.
• Мы идëм в музей. = We're going to the museum.

 3. It is used with other prepositions, mainly через to express movement taking place inside a location, “along”, and за to express the figurative movement of swapping or exchanging.

• Мы идëм через лес. = We're walking through the woods.
• Мы идëм через улыцу.  = We're going across the street.
• Спасибо за письмо. = Thanks for the letter.
• Cколько получает писатель за книгу? = How much does the author get for a book?
• Я – за Путина! = I'm for Putin1

Forming the accusative case not terribly tricky, except in the case of animate nouns. (See this post.)

For neuter nouns and inanimate masculine nouns, the accusative is the same as the nominative. (Note: neuter animate nouns are indeclinable; that’s why they don’t have a special case ending. They (preview of coming attractions!) will take animate-type adjective endings.)

Я вижу сад = I see a garden.
Я вижу лес = I see the woods.
Я вижу музей = I see a museum.
Я вижу рубль = I see a ruble.

Feminine and masculine nouns ending in A or Я take У or Ю for their accusative case ending. Those ending in Ь do not change.

Я вижу папу = I see Daddy.
Я вижу улыцу = I see the street.
Я вижу армию = I see an army.
Я вижу мать = I see the mother.

Animate masculine nouns take A or Я for their ending, depending on whether the consonant they end in is hard (A) or soft (Ь and Й are replaced by Я, or, more accurately, Й/Ь + А = Я). This is actually the same ending as the genitive case, which we haven't met yet; you're getting a leg up here: inanimate masculines have the same genitive forms.

Я вижу героя = I see a hero.
Я вижу Андрея = I see Andrey.
Я вижу коня = I see a horse.
Я вижу человека = I see a person.
Я вижу Путина = I see Putin.