Learn Hardcore Russian: Она наверняка уже дома, потому что свет в её комнате горит. - She is surely already at home because the light in her room is on.

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

Why is дома used instead of в доме?

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.

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

Why is there no verb meaning is in Она наверняка уже дома?

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

So:

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

What does наверняка mean here?

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

In this sentence, it suggests strong confidence:

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