Вечером в коридоре темнее, чем на кухне.

In Russian, the instrumental case is often used to express time when with parts of the day and some other time words.

So:

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

So вечером here means in the evening, not by means of the evening or anything literal like that. This is just a standard time expression.

This is one of those places where Russian uses different prepositions based on idiom, not pure logic.

• в коридоре = in the corridor
• на кухне = in the kitchen

For many locations, Russian simply has a conventional choice of preposition:

• в комнате = in the room
• в доме = in the house
• на кухне = in the kitchen
• на улице = in the street / outside
• на работе = at work

So you usually just learn the noun together with its usual preposition.

Both коридоре and кухне are in the prepositional case because they follow в or на in a location meaning.

Темнее is the comparative form of тёмный = dark.

• тёмный = dark
• темнее = darker

In this sentence, it works like it is darker. Russian often uses the comparative this way without an explicit subject.

So:

• В коридоре темнее. = It is darker in the corridor.
• Здесь теплее. = It is warmer here.
• Там интереснее. = It is more interesting there.

This is very common in Russian.

Russian normally omits the verb to be in the present tense.

So where English says:

• It is darker in the corridor

Russian can simply say:

• В коридоре темнее

There is no present-tense есть here. That is normal.

So the full sentence literally looks something like:

• In the evening, in the corridor, darker than in the kitchen

But naturally it means:

• In the evening, it is darker in the corridor than in the kitchen.

Чем means than in comparisons.

So:

• темнее, чем на кухне = darker than in the kitchen

This is the normal word used after a comparative:

• больше, чем... = more than...
• меньше, чем... = less than...
• лучше, чем... = better than...
• темнее, чем... = darker than...

Russian often omits repeated material when it is obvious from context.

So:

• Вечером в коридоре темнее, чем на кухне.

literally compares:

• In the evening, it is darker in the corridor than it is in the kitchen.

But Russian does not need to repeat темнее or add a full clause after чем. The second part can be shortened to just the place phrase:

• чем на кухне = than in the kitchen

This kind of ellipsis is very common.

They are both in the prepositional case, used after в and на when talking about location.

Base forms:

• коридор → в коридоре
• кухня → на кухне

So the endings change because the sentence is talking about where something is darker.

Compare:

• в коридор = into the corridor (movement, usually accusative)
• в коридоре = in the corridor (location, prepositional)

and

• на кухню = to the kitchen
• на кухне = in the kitchen

Yes, the word order could be different. Russian word order is more flexible than English.

Starting with Вечером puts the time frame first:

• Вечером в коридоре темнее, чем на кухне.

This sounds natural because it sets the scene: in the evening.

Other orders are possible, for example:

• В коридоре вечером темнее, чем на кухне.
• Темнее вечером в коридоре, чем на кухне.

But the original version is the most straightforward and natural.

In Russian, word order often reflects focus and what the speaker wants to highlight, not just grammar.

Russian usually prefers the simple comparative form when possible.

So instead of a longer form like более тёмный, Russian normally says:

• темнее = darker

This is shorter and more natural here.

Also, более тёмный would usually describe a noun directly, like:

• более тёмный коридор = a darker corridor

But in this sentence, the idea is not a darker corridor as a noun phrase. It is it is darker in the corridor as a complete statement. That is why темнее is the right form.

This is a spelling convention. In Russian, ё is often written as е in ordinary text, even though the pronunciation may still reflect ё in related forms.

The adjective is:

• тёмный

But the comparative is usually written:

• темнее

This is the standard spelling.

More broadly, Russian often omits the dots over ё in normal writing, although dictionaries and learner materials may include them more consistently.

Yes. English needs a dummy subject:

• It is darker...

Russian does not. This is an impersonal-style statement about the environment or conditions.

So:

• В коридоре темнее.

means:

• It is darker in the corridor.

There is no actual word for it in Russian here, but the meaning is fully complete and natural.