Ребята уже сидят в кафе и ждут нас.

Because ребята is a plural noun, even though its ending may look unusual to an English speaker.

A few helpful points:

• Ребята means something like guys, kids, or the gang in an informal way.
• In Russian, not all plural nouns end the way learners first expect. Some plural forms end in -а / -я.

• Even though it ends in -а, it is still grammatically plural, so it takes a plural verb:

  Ребята сидят = plural subject + plural verb

So сидят is correct, not сидит.

Not necessarily. Ребята is often used very broadly in informal Russian.

It can refer to:

• a group of boys
• a mixed group of people
• friends or companions in a casual tone

So depending on context, it may mean guys, everyone, you all, or the others. It is usually friendly and informal.

Because Russian often leaves pronouns out when they are not needed.

In this sentence, the subject is already clearly stated as ребята, so adding они would usually be unnecessary.

• Ребята уже сидят в кафе и ждут нас. = natural
• Они уже сидят в кафе и ждут нас. = also possible, but only if the context already makes it clear who they are
• Ребята, они уже... would usually sound redundant unless you want emphasis

Russian commonly avoids repeating information that is already obvious.

Because Russian does not have a separate present continuous form like English am sitting / are waiting.

Instead, Russian uses the regular present tense for both:

• general present: they sit
• ongoing action now: they are sitting

So:

• сидят can mean sit or are sitting
• ждут can mean wait or are waiting

The context tells you which English translation sounds natural. In this sentence, the idea is clearly something happening right now, so English usually uses are sitting and are waiting.

Уже means already or by now.

It adds the idea that the situation has begun before the moment we are talking about, or earlier than someone may have expected.

So Ребята уже сидят в кафе suggests something like:

• they are already at the café
• they got there before us
• by this point, they are there and seated

It often gives a slight sense of the action has happened / the state is in place now.

Here в кафе means in the café, so в is being used with location.

Normally, with location, в takes the prepositional case. But кафе is a special kind of noun:

• it is an indeclinable borrowed word
• its form does not change across cases

So even though the grammar requires the prepositional idea, the word still looks the same:

• в кафе
• о кафе
• из кафе

The form stays кафе.

Because ждать usually takes a direct object, and here us is the thing being waited for.

The pronoun мы changes form:

• nominative: мы = we
• object form: нас = us

So:

• мы ждём их = we are waiting for them
• они ждут нас = they are waiting for us

For a learner, the main thing to remember is:

• after ждать, you normally use the object form
• нас is the correct form for us

Because Russian ждать works differently from English wait for.

In English, you need:

• wait for someone

In Russian, the verb itself directly takes the object:

• ждать кого? что?
• ждать нас = literally to wait us, but naturally it means to wait for us

So the idea of for is built into how the Russian verb works.

Both are 3rd person plural present-tense verb forms.

• сидят comes from сидеть = to sit
• ждут comes from ждать = to wait

Because the subject is plural, both verbs are plural too:

• ребята сидят
• ребята ждут

This sentence has one subject, ребята, and two coordinated verbs:

• сидят
• ждут

So the structure is basically:

[subject] + [adverb] + [verb] + [place] + and + [verb] + [object]

Because и is simply joining two verbs that share the same subject:

• сидят
• ждут

The subject ребята applies to both verbs, so this is a normal compound predicate, and no comma is needed.

So Russian writes:

Ребята уже сидят в кафе и ждут нас.

A comma would usually be wrong here.

Russian word order is more flexible than English word order.

This sentence uses a very natural, neutral order:

Ребята уже сидят в кафе и ждут нас.

That order presents the information smoothly:

• ребята = who
• уже = already
• сидят в кафе = where they are
• и ждут нас = what else they are doing

You can move уже in some contexts, but that changes emphasis:

• Уже ребята сидят в кафе... = more marked, with emphasis
• Ребята сидят уже в кафе... = possible, but the focus shifts

So the original word order is the most neutral and natural for ordinary speech.