Eto autobusa.

Here, eto is a presenting / pointing-out word. It is used when you notice something or draw someone’s attention to it, often with a sense like:

• there is ...
• there’s ...
• here comes ...
• look, there’s ...

So Eto autobusa. is a natural way to point out the bus when it appears or becomes relevant.

Because after evo / eto / eno, Serbian very often uses the genitive case.

So:

• autobus = nominative singular
• autobusa = genitive singular

That is why the sentence is Eto autobusa., not Eto autobus.

This is a very common pattern:

• Evo kuće. = Here’s the house.
• Eto Marka. = There’s Marko.
• Eno psa. = There’s the dog over there.

Yes, from an English point of view, it may feel like something is omitted.

Serbian often uses short expressions like this without an explicit verb, especially when pointing something out. English usually needs a verb:

• There is the bus.
• Here comes the bus.
• There’s the bus.

In Serbian, Eto autobusa. is complete by itself.

It can cover both ideas, depending on context.

Common natural translations are:

• There’s the bus.
• Here comes the bus.
• Look, the bus is here.

If you are waiting for the bus and it appears, Eto autobusa. often feels very much like Here comes the bus or There’s our bus.

These three words are related and often translated similarly, but they differ in distance / perspective:

• evo = here, near the speaker
• eto = there / here you are, often near the listener or just being pointed out
• eno = over there, farther away from both speaker and listener

Very roughly:

• Evo autobusa. = Here’s the bus.
• Eto autobusa. = There’s the bus.
• Eno autobusa. = There’s the bus over there.

In real speech, the boundaries are not always perfectly strict, and eto is often used quite broadly when drawing attention to something.

In this kind of sentence, it usually feels like the bus in English, especially if both people know which bus is being talked about.

So in context, Eto autobusa. will often be understood as:

• There’s the bus.
• Here comes the bus.

Serbian has no articles like a and the, so definiteness is understood from context.

Not in that form.

With full nouns after eto, Serbian normally uses the genitive:

• Eto autobusa.

If you replace the noun with a pronoun, Serbian often uses a clitic pronoun instead:

• Eto ga. = There it is. / There he is.
• Eto je. = There she/it is.
• Eto ih. = There they are.

So you either say:

• Eto autobusa. or
• Eto ga.

but not Eto ga autobus.

It is very normal and natural in everyday speech. It is not slang, and it is not especially formal either. It is a common conversational way to point something out quickly.

For example, while waiting at a stop, someone might say:

• Eto autobusa.

That sounds completely natural.

This short expression is normally said as:

• Eto autobusa.

That is the standard, natural order for this kind of presenting phrase. Since eto introduces what you are pointing out, it usually comes first.

They mean different things.

• To je autobus. = That is a bus.
  This identifies something.

• Eto autobusa. = There’s the bus. / Here comes the bus.
  This points something out as it appears or becomes noticeable.

So To je autobus is more like naming or identifying, while Eto autobusa is more like drawing attention to it.