Днём я обычно работаю, а вечером отдыхаю.

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Breakdown of Днём я обычно работаю, а вечером отдыхаю.

• вечером: in the evening
• работать: to work
• а: and

Questions & Answers about Днём я обычно работаю, а вечером отдыхаю.

Why is днём used here instead of день or в день?

Днём is the instrumental case of день (day), used adverbially to answer the question “когда?” (when?).

In Russian, times of day often use the instrumental without a preposition to mean “in the … / during the …”:

• утром – in the morning
• днём – in / during the day
• вечером – in the evening
• ночью – at night

So днём я обычно работаю literally means “by day I usually work”, but the natural English is “In the daytime I usually work” or “During the day I usually work.”
В день exists, but it usually means “per day” (e.g. два часа в день – two hours per day), not “in the day” as a time reference.

What case is днём exactly, and why does день change to дн-?

Днём is instrumental singular:

• Nominative: день (day)
• Instrumental: днём (by/with/at/in the day)

The change день → дн- is a stem alternation that happens in some Russian nouns. You just have to memorize that день has the stem дн- in oblique cases (дня, дню, днём, дне).

So днём is just the regular instrumental form of день, even though the stem looks a bit different.