Learn Hardcore Russian: Вечером свет в комнате включается автоматически, когда я вхожу. - In the evening the light in the room turns on automatically when I enter.

Вечером is the instrumental singular form of вечер (“evening”), and in Russian this form is very often used adverbially to mean “in the evening”.

утром – in the morning
днём – in/at daytime
вечером – in the evening
ночью – at night

You could also say:

по вечерам – “in the evenings / every evening” (emphasizes repetition)

Here, вечером simply sets the time frame: “In the evening, …”. Putting it at the very beginning is a common, natural way to start a sentence with a time expression in Russian.

Why doesn’t Russian say “the light” — why just свет without an article?

Russian has no articles (“a/an/the”). Whether English would say “light” or “the light”, Russian simply uses свет.

Context tells us whether it’s “light in general” or “the (specific) light”:

Свет в комнате включается автоматически – The light in the room turns on automatically.
Here it’s clearly the light in that room (because it’s limited by в комнате), even though Russian doesn’t mark definiteness.

So свет в комнате is understood as “the light in the room,” not “a light.”

Why is свет the subject of включается? In English I might say “Someone turns on the light.”

Russian is very comfortable making inanimate things the subject when something happens to them:

Свет включается – The light turns on (literally: “the light switches itself on”)
Дверь открывается – The door opens
Окно закрывается – The window closes

When an automatic system does something, Russian usually describes it as if the object is the one acting (with a reflexive verb). If you wanted to emphasize a person doing it, you’d say:

Я включаю свет. – I turn on the light.

What exactly does the reflexive verb включается mean, and how is it different from включают or включают свет?

Включается = включает + -ся, literally “switches (itself) on.” The -ся ending is the reflexive marker, and in this sentence it works like a middle/passive form:

Свет включают автоматически. – They turn on the light automatically. (someone unspecified does it)
Свет включается автоматически. – The light turns on automatically. (natural, neutral, no agent mentioned)

So:

включают свет – active, someone (they/people) turns it on.
свет включается – the light “gets turned on / turns itself on,” which nicely fits the idea of automation.

Why is включается in the present tense, not включится (future)?

In Russian, the present tense of an imperfective verb (like включаться) is used not only for “right now,” but also for repeated, habitual actions:

Вечером свет в комнате включается автоматически.
→ In the evening, the light in the room turns on automatically (every time that situation happens).

If you used the perfective future включится, it would sound like a one-time, future event:

Сегодня вечером свет в комнате включится автоматически.
→ This evening the light will come on automatically (this one time).

So включается here matches the general, habitual meaning of the sentence.

Why is it я вхожу, not я войду?

Вхожу is present tense imperfective of входить (“to enter, to go in”).

Войду is future tense perfective of войти (“to enter (once), to step in”).

In this sentence, we’re talking about what usually happens whenever you enter:

…когда я вхожу. – when I enter (whenever I enter).

If you said:

…когда я войду.

it would sound like a concrete, one-time future event: “when I (will) enter (on that specific occasion).” That doesn’t match the general rule-like description here.

Could I say когда я вхожу в комнату? Why is в комнату missing?

Yes, you absolutely can:

Вечером свет в комнате включается автоматически, когда я вхожу в комнату.

That’s fully correct and explicit.

In the original, в комнату is omitted because it’s obvious from context: we already have свет в комнате, so it’s clear which place you’re entering. Russian often drops repeated information if it’s perfectly clear:

Я открываю дверь и вхожу. – “I open the door and go in (there).”
“Into where?” is understood from the situation.

What’s the difference between вхожу, захожу, and прихожу? Could I use one of the others?

All three involve movement toward something, but they’re not interchangeable:

вхожу (входить) – to enter (cross a boundary; go inside a room/building).
Focus: crossing the threshold.
захожу (заходить) – to drop in / pop in / step in (for a bit) or to go behind something, depending on context.
Заходить в комнату can work, but it often has a nuance of “going in (maybe briefly / on the way).”
прихожу (приходить) – to arrive (on foot) somewhere.
Прихожу домой – I come home, not I go into home.

In this sentence, you really want the idea of entering the room, so вхожу is the most precise and neutral choice.

Why is there a comma before когда я вхожу?

In Russian, a clause introduced by когда (“when”) is a subordinate clause, and it is normally separated by a comma:

Свет включается автоматически, когда я вхожу.
(main clause) , (subordinate “when” clause)

The rule is: main clause + когда + subordinate clause → put a comma before когда.

You can also reverse the order:

Когда я вхожу, свет в комнате включается автоматически.

Then the comma comes after the когда-clause instead.

How flexible is the word order? Could I move вечером or автоматически?

Russian word order is fairly flexible, and this sentence can be rearranged without changing the core meaning, though the emphasis shifts slightly. All of these are grammatical:

Вечером свет в комнате включается автоматически, когда я вхожу.
(time at the start; neutral)
Свет в комнате вечером включается автоматически, когда я вхожу.
(slight extra focus on “the light in the room, in the evening…”)
Свет в комнате включается автоматически вечером, когда я вхожу.
(a bit more emphasis on “automatically in the evening” as a detail)
Свет в комнате автоматически включается вечером, когда я вхожу.
(slight emphasis on автоматически)

All are understandable; the original is very natural and neutral in tone.

Why is it автоматически and not автоматический?

Автоматический is an adjective: “automatic.”
Автоматически is an adverb: “automatically.”

Here we describe how the light turns on (the manner of the action), so we need an adverb:

включается автоматически – turns on automatically.

If you used the adjective, you would have to modify a noun:

автоматический свет – automatic light
автоматическое включение света – automatic switching-on of the light

Why is it в комнате and not на комнате?

Russian uses в for being inside closed or defined spaces, and на for surfaces or some open areas / events / institutions. For locations like rooms, houses, buildings, you normally use в:

в комнате – in the room
в доме – in the house
в классе – in the classroom

На комнате would be wrong here; it would sound like “on the room” (on top of it), which makes no sense.