Я ставлю книгу в шкаф.

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Breakdown of Я ставлю книгу в шкаф.

• книга: the book
• в: into
• шкаф: the cupboard

Questions & Answers about Я ставлю книгу в шкаф.

Why is книгу used instead of книга?

Книгу is in the accusative case, which is used for the direct object of the verb (the thing directly affected by the action).

• Nominative (dictionary form): книга – a book (subject)
• Accusative: книгу – a book (object)

In this sentence, the book is what you are putting, so it must be in the accusative:
Я ставлю книгу в шкаф. – I put the book in the cupboard.

What case is шкаф in, and why does it look like the dictionary form?

Шкаф here is also accusative, because it is the destination of movement (where you are putting the book).

For inanimate masculine nouns like шкаф, the accusative form is identical to the nominative:

• Nominative: шкаф – a cupboard
• Accusative: шкаф – into the cupboard (after в with motion)

So the word looks the same, but functionally it is accusative.

Why is it в шкаф and not в шкафу?