Он наверняка уже дома, потому что свет в его комнате горит.

Дома means at home and is a very common adverb in Russian.

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

So in your sentence, дома is the natural choice because the idea is he’s home, not just physically inside a building.

In the present tense, Russian usually does not use a verb equivalent to is/am/are.

So:

• Он дома literally looks like He home
• but it means He is at home

This is completely normal in Russian. The present-tense form of to be is usually omitted.

Наверняка means something like:

• for sure
• certainly
• almost certainly

In this sentence, it expresses a confident conclusion based on evidence. The speaker is not just guessing randomly; they are reasoning from the fact that the light is on.

A rough English feel might be:

• He’s almost certainly home
• He must already be home

This is a very common question.

• наверное = probably / I suppose
• наверняка = for sure / almost certainly / definitely

So наверняка sounds stronger and more confident than наверное.

Compare:

• Он, наверное, дома. = He’s probably at home.
• Он наверняка дома. = He’s almost certainly / definitely at home.

In your sentence, наверняка fits well because the speaker has evidence: the light is on.

Уже means already.

So:

• Он дома = He is home
• Он уже дома = He is already home

It suggests that he has arrived by now. It often implies a contrast with an earlier time when he was not home yet.

Yes, Russian word order is flexible.

Он наверняка уже дома is natural and neutral. But other orders are also possible, with slightly different emphasis:

• Он уже наверняка дома — puts a bit more focus on already
• Наверняка он уже дома — starts with the speaker’s certainty
• Он дома уже наверняка — possible in speech, but less neutral

Russian word order often reflects emphasis, information structure, or style more than basic grammar.

Because потому что introduces a subordinate clause meaning because.

So the sentence has two parts:

• Он наверняка уже дома
• потому что свет в его комнате горит

In Russian, a comma is normally used before потому что in this kind of sentence.

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

So:

• комната = nominative
• в комнате = prepositional, meaning in the room

And его means his.

So:

• в его комнате = in his room

Because свой normally refers to the subject of its own clause.

In the clause:

• свет в его комнате горит

the grammatical subject is свет (the light), not он.

If you used своей here, it would try to refer back to свет, which does not make sense: the light in its own room.

So you use его to show that the room belongs to him from the main clause:

• в его комнате = in his room

In Russian, свет often works like the light in a general sense, and it is very natural to say:

• Свет горит. = The light is on.

Even if English might say the lights are on, Russian commonly uses singular свет. It refers to the room being lit, not necessarily to one specific lamp.

Right — but this is the normal Russian expression.

The verb гореть literally means to burn, but it is also used for lights, lamps, candles, etc. to mean:

• to be on
• to be lit

So:

• Свет горит. = The light is on.
• Лампа горит. = The lamp is on / lit

This is standard Russian usage.

Russian often places the most important or newest information toward the end of the clause.

In:

• свет в его комнате горит

the final горит completes the key point: the light is on.

But other word orders are also possible, for example:

• В его комнате горит свет.

This is also very natural. It can sound a bit more like There is a light on in his room or put slightly different focus on the location.

So the meaning stays very close, but the emphasis can shift.

Yes, absolutely. That version is very natural too:

• Он наверняка уже дома, потому что в его комнате горит свет.

Both are correct:

• свет в его комнате горит
• в его комнате горит свет

The difference is mainly in emphasis and flow, not basic meaning.

• свет в его комнате горит focuses a little more on the light
• в его комнате горит свет starts with the location in his room

It expresses deduction based on evidence.

The speaker does not literally know for an absolute fact that he is home. Instead, they see that the light in his room is on and draw a strong conclusion.

That is why наверняка works well here: it signals a high degree of confidence based on what the speaker observes.