Я аккуратно положил книгу в сумку.

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Breakdown of Я аккуратно положил книгу в сумку.

• книга: the book
• сумка: the bag
• положить: to put

Questions & Answers about Я аккуратно положил книгу в сумку.

Why is it положил, and what form is that?

Положил is the past tense, masculine singular form of the verb положить.

• положить = to put / to place
• положил = put / placed (said by a male speaker)

Russian past tense often shows gender in the singular:

• положил = a man said it
• положила = a woman said it
• положило = neuter
• положили = plural

So Я аккуратно положил книгу в сумку would normally be said by a male speaker.

Why is it книгу and not книга?

Because книгу is the accusative singular form of книга.

Here, книга is the direct object of the verb положил: it is the thing being put somewhere.

• nominative: книга = book
• accusative: книгу = book as the object of the action

For many feminine nouns ending in -а, the accusative singular changes to -у:

• книга → книгу
• сумка → сумку
• ручка → ручку

So положил книгу means put the book.