Learn Russian: У мальчика есть велосипед, а у девочки есть гитара. - The boy has a bicycle, and the girl has a guitar.

In Russian, possession is usually expressed by saying "By [someone] there is [something]." The preposition у means "by" or "at" and requires the owner to be in the Genitive case (мальчика, девочки). The word есть means "is" or "there is".

Why does мальчик change to мальчика?

Because it follows the preposition у, which always requires the Genitive case. For masculine nouns that end in a consonant like мальчик, you form the Genitive case by adding -а to the end, making it мальчика.

Why is the ending for девочка spelled девочки instead of девочкы?

Normally, feminine nouns ending in -а change to -ы in the Genitive case. However, Russian has a strict spelling rule: you can never write the letter ы after г, к, х, ж, ч, ш, or щ. Because the stem of девочка ends in -к, you must use -и instead, resulting in девочки.

Why aren't велосипед and гитара in the Accusative case?