Она наверняка уже дома, потому что свет в её комнате горит.

Дома is a very common Russian adverb meaning at home. In this sentence, Она уже дома means She is already home / at home.

Russian often uses дома where English uses at home, instead of saying в доме.

• дома = at home
• в доме = in the house/building

So Она дома means she is at her home, while Она в доме would focus more on her being inside a particular house.

In Russian, the verb to be is usually omitted in the present tense.

So:

• Она дома literally looks like She home
• but it means She is home

This is completely normal in Russian. You only use forms of быть more clearly in the past or future:

• Она была дома = She was at home
• Она будет дома = She will be at home

Наверняка means something like for sure, almost certainly, or very likely depending on context.

In this sentence, it suggests strong confidence:

• Она наверняка уже дома = She is almost certainly home already

It is stronger than a vague maybe. It often implies the speaker has a good reason to think so.

This is a very common question.

• наверное = probably, I suppose, likely
• наверняка = almost certainly, definitely/probably with confidence

So наверняка sounds more certain than наверное.

Compare:

• Она, наверное, дома. = She is probably at home.
• Она наверняка дома. = She is almost certainly at home.

In your sentence, the reason comes right after it: the light is on in her room. That makes наверняка very natural.

Уже means already.

So:

• Она дома = She is home
• Она уже дома = She is already home

It suggests that she has arrived by now, perhaps earlier than expected or by the time the speaker is talking.

Потому что means because.

It introduces the reason:

• Она наверняка уже дома, потому что свет в её комнате горит.
• She is almost certainly already home, because the light in her room is on.

It is a fixed expression and is always written as two words.

Russian normally uses a comma before потому что when it introduces a subordinate clause, just like English often uses a comma before because in some styles.

So the sentence is divided into:

• main clause: Она наверняка уже дома
• reason clause: потому что свет в её комнате горит

That is why the comma is there.

Because after в when it means location, Russian uses the prepositional case.

The base form is:

• комната = room

But after в meaning in, it changes:

• в комнате = in the room

So:

• в её комнате = in her room

This is a normal case change for feminine nouns ending in -а.

Because её here means her as a possessive pronoun, and in this use it is indeclinable. That means it does not change form for case, gender, or number.

So you get:

• её комната = her room
• в её комнате = in her room
• из её комнаты = from her room

The noun changes, but её stays the same.

Both spellings are common in modern Russian writing:

• её
• ее

They represent the same word. The version with ё shows the pronunciation more clearly and is often helpful for learners. In many printed texts, Russians omit the dots and write ее, but the meaning is still её.

So in this sentence, её комнате and ее комнате mean the same thing.

Here, свет means the light, and горит literally means is burning.

In Russian, свет горит is the normal way to say:

• the light is on

So this part:

• свет в её комнате горит

literally means:

• the light in her room is burning

but naturally in English it means:

• the light in her room is on

Russian often uses гореть for lights, lamps, candles, and anything that shines as if burning.

Examples:

• Свет горит. = The light is on.
• Лампа горит. = The lamp is on.
• Свеча горит. = The candle is burning.

So although гореть literally means to burn, with lights it often corresponds to English to be on.

Russian word order is flexible, so горит can appear at the end without sounding strange.

The order свет в её комнате горит is natural because it first introduces what light we are talking about, and then gives the key information at the end: it is on.

You could also hear:

• В её комнате горит свет.

That also means The light is on in her room, but the focus is slightly different. Russian word order often shifts for emphasis rather than basic grammar.

Yes. Russian allows several word orders, especially in speech.

For example:

• Она уже наверняка дома, потому что в её комнате горит свет.
• Наверняка она уже дома, потому что свет в её комнате горит.

These versions still mean basically the same thing, but the emphasis shifts a little.

Your original sentence sounds natural and balanced:

• first the conclusion: She is almost certainly already home
• then the reason: because the light in her room is on

In this sentence, она is natural and useful because it makes the subject clear: she.

Russian can sometimes omit pronouns when the meaning is obvious from context, but with a present-tense sentence like this, keeping она is usually clearer.

So:

• Она наверняка уже дома = clear and natural
• Наверняка уже дома = possible only in a context where everyone already knows who is being talked about

It is mostly neutral everyday Russian.

Nothing in it is especially slangy or formal:

• наверняка is common
• уже дома is everyday language
• потому что is neutral
• свет горит is a normal expression

So this is a very natural sentence for ordinary spoken or written Russian.