Девушка читает книгу в парке.

the book

Breakdown of Девушка читает книгу в парке.

• читать: to read
• парк: the park
• в: in

Questions & Answers about Девушка читает книгу в парке.

Why does книга change to книгу in this sentence?

Книгу is the accusative case form of книга.

• The accusative case is used for the direct object of the verb – the thing that is being acted upon.
• The verb читать (to read) takes a direct object (what is being read).
• In Девушка читает книгу, the girl is reading what? → книгу.

For feminine nouns ending in -а or -я (like книга, машина, газета), the accusative singular is usually formed by changing:

• -а → -у: книга → книгу, машина → машину
• -я → -ю: статья → статью, история → историю

Why is it в парке and not в парк or на парке?

The preposition в can take either the prepositional or accusative case, depending on meaning:

• в парке (prepositional case) = in the park (location, where?)
• в парк (accusative case) = into the park (direction, where to?)

In this sentence, the girl is already in the park, reading there, so we talk about location, not movement:

• Где девушка читает книгу? – В парке. → prepositional case парке.

Why not на парке?