Вечером я вспоминаю наш отпуск.

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Breakdown of Вечером я вспоминаю наш отпуск.

• вечером: in the evening
• отпуск: the vacation
• наш: our

Questions & Answers about Вечером я вспоминаю наш отпуск.

What case is вечером, and why does it end in -ом instead of using a preposition like в вечер?

Вечером is the instrumental case of вечер (evening):

• вечер → вечером (instrumental singular)

In Russian, the instrumental is very often used to express “when?” for parts of the day:

• утро → утром – in the morning
• день → днём – in/at daytime
• вечер → вечером – in the evening
• ночь → ночью – at night

So instead of saying в вечер, Russian uses this instrumental form without a preposition. It functions almost like an adverb meaning “in the evening”.

Could I change the word order and say Я вспоминаю наш отпуск вечером? Is there any difference?

Yes, Я вспоминаю наш отпуск вечером is also correct.

Both sentences are fine:

• Вечером я вспоминаю наш отпуск.
• Я вспоминаю наш отпуск вечером.

The difference is mostly in emphasis:

• Starting with Вечером puts more focus on when: As for the evening, that’s when I remember our vacation.
• Putting вечером at the end sounds a bit more neutral in English: I remember our vacation in the evening.

Grammatically they are equivalent; Russian word order is flexible, and here both are natural.