Зимой темнеет рано, поэтому я включаю свет в комнате.

Зимой is the instrumental case of зима (winter).

In Russian, names of seasons are very often used in the instrumental case to mean during that season:

• зимой = in winter
• летом = in summer
• осенью = in autumn
• весной = in spring

So Зимой here means in winter / during winter, not just winter as a dictionary form.

Because темнеет is being used impersonally.

Russian often uses certain verbs without a stated subject when talking about weather, time, or general conditions. Here темнеет means it gets dark or it is getting dark, even though Russian does not need a word for it.

So:

• Темнеет. = It’s getting dark.
• Зимой темнеет рано. = In winter, it gets dark early.

This is very natural in Russian.

Темнеет comes from темнеть, which means to get dark or to grow dark.

So it describes a process:

• темно = dark
• темнеет = it is becoming dark / it gets dark

In this sentence, it means that darkness comes early in winter.

Because рано is an adverb, and it modifies the verb темнеет.

You need an adverb here because the sentence is describing how early it gets dark:

• рано = early
• поздно = late

Compare:

• ранний вечер = an early evening (ранний is an adjective)
• темнеет рано = it gets dark early (рано is an adverb)

Поэтому means therefore, that’s why, or so.

It connects the first idea with the result:

• Зимой темнеет рано = In winter it gets dark early
• поэтому я включаю свет в комнате = therefore / so I turn on the light in the room

The comma is used because Russian separates these two clauses here:

• cause: Зимой темнеет рано
• result: поэтому я включаю свет в комнате

So the comma works much like in English when you separate two related clauses.

Включаю is present tense and usually suggests a repeated, habitual action here:

• я включаю свет = I turn on the light / I switch on the light

In this sentence, the meaning is general: when it gets dark early in winter, this is what I do.

If you said я включу свет, that would usually mean I will turn on the light (a single future action), which does not fit the general statement as well.

So:

• включаю = I turn on / I am turning on
• включу = I will turn on

Because свет is a masculine inanimate noun, and in Russian the accusative singular of masculine inanimate nouns is usually the same as the nominative singular.

So:

• nominative: свет
• accusative: свет

Since включать takes a direct object, свет is in the accusative, but it looks exactly the same as the dictionary form.

Usually it means turn on the light in a general sense.

Russian often uses свет the way English uses the light as a general household concept. It does not always focus on the number of lamps or bulbs.

So включаю свет naturally means:

• I turn on the light
• sometimes more generally, I switch the lights on

If you specifically wanted to talk about multiple lights, you could use огни or another more specific noun, but свет is the normal everyday choice here.

Because в + prepositional case is used to mean in a place.

Here:

• комната = room
• prepositional singular: в комнате = in the room

So the phrase tells you where the light is being turned on.

Compare:

• в комнате = in the room
• в доме = in the house
• в школе = at school / in the school

Yes. Зимой рано темнеет is also perfectly natural.

Russian word order is more flexible than English, because case endings carry a lot of the grammatical information. The order can shift for emphasis or style.

These are all natural:

• Зимой темнеет рано.
• Зимой рано темнеет.

The meaning stays basically the same. The original sentence sounds very neutral and natural.

Russian has no articles like a or the.

So в комнате can mean:

• in the room
• in a room

The exact meaning depends on context. In this sentence, English usually translates it as in the room, because that sounds most natural in context.

Yes, you could say:

• Зимой рано становится темно.

This also means In winter it gets dark early.

The difference is mostly stylistic:

• темнеет = it gets dark, a bit more compact and verb-like
• становится темно = it becomes dark, slightly more descriptive

Both are correct and natural.

Adding в комнате makes the location explicit: it is the light in the room that is being turned on.

Without it, the sentence would still work:

• Зимой темнеет рано, поэтому я включаю свет.

That means In winter it gets dark early, so I turn on the light.

Adding в комнате just gives a bit more detail.