Брат любит омлет с грибами, а сестра — кашу с персиком.

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Breakdown of Брат любит омлет с грибами, а сестра — кашу с персиком.

• с: with
• сестра: the sister
• брат: the brother

Questions & Answers about Брат любит омлет с грибами, а сестра — кашу с персиком.

Why is каша written as кашу, but омлет stays омлет?

Because both words are direct objects of любит, so they must be in the accusative case.

• омлет is a masculine inanimate noun, and for masculine inanimate nouns, the accusative singular is usually the same as the nominative singular.
  • омлет → омлет
• каша is a feminine noun ending in -а, and its accusative singular changes to -у.
  • каша → кашу

So:

• любит омлет
• любит кашу

are both exactly what Russian grammar requires.

Why are грибами and персиком in those forms?

Because they come after the preposition с, meaning with, and in this meaning с normally requires the instrumental case.

So:

• грибы → грибами = with mushrooms
• персик → персиком = with peach

This is a very common pattern in Russian:

• чай с лимоном = tea with lemon
• хлеб с маслом = bread with butter
• салат с помидорами = salad with tomatoes