Осенью тёплый день часто переходит в прохладный вечер.

In autumn a warm day often turns into a cool evening.

Breakdown of Осенью тёплый день часто переходит в прохладный вечер.

• день: the day
• вечер: the evening
• прохладный: cool
• часто: often

Questions & Answers about Осенью тёплый день часто переходит в прохладный вечер.

What does осенью literally mean, and what case is it in?

Осенью means “in (the) autumn / in fall” here.

Grammatically, осенью is:

• the instrumental singular of осень (autumn)
• used adverbially to show time: “in autumn / during autumn”

Russian often uses the instrumental case to express time like this:

• зимой – in (the) winter
• весной – in (the) spring
• летом – in (the) summer
• осенью – in (the) autumn

So осенью in this sentence answers the question когда? (when?): When does a warm day often turn into a cool evening? – Осенью.

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Why is there no preposition like “в” before осенью? Why not в осенью?

В осенью is incorrect in standard Russian.

When talking generally about the season as a time frame, Russian usually:

• uses just the instrumental form without a preposition:
  • осенью – in autumn
  • зимой – in winter
  • летом – in summer
  • весной – in spring

Using в + осенью would be a double marker (“in + in autumn”) and sounds ungrammatical.

You can see the same pattern in simple sentences:

• Осенью часто идёт дождь. – It often rains in autumn.