Після уроку я кладу зошит у рюкзак.

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Breakdown of Після уроку я кладу зошит у рюкзак.

• у: in
• урок: the lesson
• після: after

Questions & Answers about Після уроку я кладу зошит у рюкзак.

Why is it після уроку and not після урок?

Because після always requires the genitive case.

• Base form: урок = lesson, class
• Genitive singular: уроку

So:

• після уроку = after the lesson / after class

This is a very common pattern in Ukrainian:

• після обіду = after lunch
• після роботи = after work
• після школи = after school

Why is уроку in the genitive, but зошит is not changed?

They are doing different jobs in the sentence.

• уроку comes after the preposition після, which requires the genitive
• зошит is the direct object of the verb кладу, so it is in the accusative

But here is the important detail: for many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

So:

• Nominative: зошит
• Accusative: зошит

It has not stayed unchanged because it has no case; it is actually accusative, but the form happens to be the same.

Why is it у рюкзак, not ?