Днём в комнате светлее, чем вечером.

In Russian, the present-tense copula to be is usually omitted.

So instead of saying something like it is lighter, Russian often just says светлее.

This sentence is an impersonal statement, so Russian does not need a subject like it either. The sense is simply:

• Днём — during the day
• в комнате — in the room
• светлее — is lighter / brighter
• чем вечером — than in the evening

So the full idea is there even without a separate word for is.

Светлее is the comparative form meaning lighter / brighter.

It comes from светлый (light, bright) and is closely related in use to светло (it is light / bright).

In this sentence, светлее works like a predicate: it describes the general lighting condition, not a noun directly. So it means something like:

• it is brighter
• it is lighter
• it is less dark

Here, the natural English sense is usually brighter rather than a lighter color.

Not here. In this sentence, светлее is not describing a specific noun like комната as an adjective would. It is being used predicatively, more like it is brighter.

Compare:

• светлая комната = a bright room
  Here светлая agrees with комната.

• в комнате светлее = it is brighter in the room
  Here светлее is not agreeing with a noun; it is just stating a condition.

That is why you see the bare comparative form.

Днём is the instrumental singular form of день (day), used adverbially.

Russian often uses the instrumental with parts of the day to mean at / in / during that time:

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

So днём here does not literally mean by the day. It is just the normal Russian way to say during the day.

For the same reason as днём. It is the standard time-expression pattern.

Вечером is the instrumental singular of вечер (evening), used to mean in the evening.

So both time words in the sentence follow a very common pattern:

• днём
• вечером

A learner should get used to these as very natural time expressions, almost like adverbs.

Historically and grammatically, it is a noun form: instrumental singular of день.

But in actual use, it often behaves like an adverbial expression of time. That is why many learners experience it as something halfway between a noun form and an adverb.

The same is true of forms like:

• утром
• вечером
• зимой
• летом

So the best practical answer is:

• grammatically: a noun form
• functionally in this sentence: an adverbial time expression

After в when it means in a place, Russian uses the prepositional case.

So:

• комната = room
• в комнате = in the room

This tells you the location where the condition is true.

Here, в комнате answers the question where?

Чем is the standard word used with comparatives to mean than.

So:

• светлее, чем вечером = brighter than in the evening

This is one of the most basic comparison patterns in Russian:

• больше, чем... — more than...
• меньше, чем... — less than...
• лучше, чем... — better than...
• светлее, чем... — brighter than...

Because in standard Russian spelling, a comparative phrase introduced by чем is normally separated by a comma.

So:

• Днём в комнате светлее, чем вечером.

That comma is the expected punctuation here.

In this sentence, it means brighter / lighter in terms of lighting, not paler in color.

Russian светлый / светлее can refer to either:

• light in color
• bright / full of light

The context tells you which one is meant. Because the sentence talks about daytime and evening, the meaning is clearly about illumination.

Yes. Russian word order is flexible.

For example, these are all possible:

• Днём в комнате светлее, чем вечером.
• В комнате днём светлее, чем вечером.
• Светлее в комнате днём, чем вечером.

The most neutral versions are the first two. Changing the order usually changes emphasis rather than core meaning.

In your sentence, starting with Днём puts the time contrast first.

Yes, that is the natural interpretation.

The phrase в комнате applies to the whole comparison: the room is brighter during the day than it is in the evening.

So the comparison is:

• same place: в комнате
• different times: днём vs вечером

It is technically possible to build a comparison analytically in some contexts, but here светлее is much more natural.

Russian usually prefers the simple comparative form when it exists:

• светлее rather than более светло
• быстрее rather than более быстро
• лучше rather than some longer alternative

So for a learner, светлее is the form to remember.

Because in ordinary Russian writing, ё is often replaced by е.

So you may see:

• днём or днем

They mean the same thing, but the pronunciation is still with ё: roughly dnyom.

For learners, it is useful to remember the true form as днём, even if native texts often write днем.