Он наверняка уже спит, потому что в его комнате темно.

Наверняка means something like for sure, certainly, almost certainly, or surely.

In this sentence, it does not mean absolute mathematical certainty. It expresses a strong conclusion based on evidence:

• Он наверняка уже спит = He’s surely/almost certainly asleep already

Compared with probably, наверняка usually sounds stronger.

• Он, наверное, уже спит = He’s probably already asleep
• Он наверняка уже спит = He’s almost certainly already asleep

So the speaker is not just guessing casually; they feel fairly confident.

Уже means already.

So:

• Он спит = He is sleeping / He sleeps
• Он уже спит = He is already sleeping / He is asleep already

It tells us that the action or state has already begun by this point. In this sentence, the idea is: based on the darkness in his room, the speaker thinks he has already gone to sleep.

In Russian, the present tense often covers both:

• simple present: he sleeps
• present continuous: he is sleeping

So он спит can mean either, depending on context.

Here, because of already and the overall situation, the natural meaning is:

• Он уже спит = He’s already sleeping / He’s already asleep

Russian does not need a separate form like English is sleeping.

The verb спать (to sleep) is normally used in the imperfective when talking about the ongoing state of sleeping.

• спит = is sleeping / asleep

That makes sense because sleeping is being presented as a state in progress, not as a completed action.

If you wanted to talk about the action of falling asleep, Russian would use a different verb:

• заснуть = to fall asleep
• Он уже заснул = He has already fallen asleep

So:

• спит = he is in the state of sleep
• заснул = he entered that state

Because потому что means because, and it introduces a subordinate clause.

So the sentence has two parts:

• Он наверняка уже спит
• потому что в его комнате темно

Russian normally puts a comma before потому что:

• ..., потому что ...

This is standard punctuation.

В его комнате means in his room.

Here, в means in, and it is used with the Prepositional case when talking about location.

• комната = room
• в комнате = in the room

So комнате is the Prepositional singular form of комната.

The phrase breaks down like this:

• в = in
• его = his
• комнате = room (Prepositional case)

Его here means his.

Unlike many other Russian words, его as a possessive does not change for case, gender, or number in this kind of use.

So you get:

• его комната = his room
• в его комнате = in his room
• у его комнаты = by his room

The noun changes, but его stays the same.

This is normal for the possessive pronouns:

• его = his
• её = her
• их = their

Russian very often expresses this idea with темно, which means dark in the sense of it is dark.

• В его комнате темно = It is dark in his room

This is a very natural Russian structure. It focuses on the condition of the space, not on describing the room as an object.

Compare:

• В комнате темно = It’s dark in the room
• Тёмная комната = a dark room

So:

• темно describes the situation or state
• тёмная is an adjective describing the noun комната

Here темно is not a regular adjective. It is a predicative word (sometimes called a category of state word).

These words are used in sentences like:

• Темно = It’s dark
• Холодно = It’s cold
• Тихо = It’s quiet
• Жарко = It’s hot

They often describe the environment, a condition, or how things feel in general, without needing a normal subject.

So в его комнате темно literally works like:

• In his room, it is dark

Yes, in a way.

In the present tense, Russian usually does not use a word for is/are in sentences like this.

English says:

• It is dark

Russian just says:

• Темно

So в его комнате темно literally looks like:

• In his room dark

but it means:

• It is dark in his room

This is completely normal in Russian.

Yes. Russian word order is fairly flexible, although different orders can slightly change the emphasis.

The neutral version here is:

• Он наверняка уже спит, потому что в его комнате темно.

Possible alternatives include:

• Он уже наверняка спит...
• Наверняка он уже спит...
• Потому что в его комнате темно, он наверняка уже спит.

These all mean roughly the same thing, but the emphasis shifts:

• putting наверняка earlier can emphasize certainty
• putting the because clause first can emphasize the reason

Still, the original sentence sounds natural and neutral.

Yes. Наверняка is fairly mobile.

For example:

• Он наверняка уже спит
• Он уже наверняка спит
• Наверняка он уже спит

All are possible.

The most natural position depends on what you want to stress. In the original sentence, наверняка comes early and nicely sets up the idea of a confident conclusion.

Потому что is the full conjunction meaning because.

• ..., потому что ... = ..., because ...

On its own, потому usually means something closer to for that reason or that’s why, and it behaves differently.

Compare:

• Он спит, потому что темно. = He is sleeping because it’s dark.
• Темно, потому он спит. = a more literary or less common structure, closer to It’s dark, so he’s sleeping or for that reason

For most learners, it is safest to treat потому что as the standard way to say because.

In English, both can fit the meaning.

• Он уже спит can be translated as:
  • He is already sleeping
  • He is already asleep

In this sentence, asleep often sounds more natural in English, because the speaker is inferring his state from the dark room.

So a very natural translation is:

• He’s almost certainly asleep already, because it’s dark in his room.

But grammatically, Russian is using the verb спать (to sleep), not a separate adjective meaning asleep.

They would usually choose a weaker word than наверняка.

For example:

• Он, наверное, уже спит... = He’s probably already asleep...
• Он, возможно, уже спит... = He may already be asleep...
• Может быть, он уже спит... = Maybe he’s already asleep...

So наверняка is one of the stronger choices for expressing confidence.