Learn Hardcore Indonesian: Dia berbicara kasar di grup chat, dan teman-teman diam saja. - He/She speaks harshly in the chat group, and the friends stay silent.

Dia can mean he, she, or even they (singular) depending on context.
Indonesian third-person pronouns are not gendered, so the sentence itself doesn’t tell you the gender. You only know from the wider story, from names mentioned earlier, or sometimes from cultural context. If you really want to specify, people might say laki-laki itu (that man) or perempuan itu (that woman) instead.

What’s the difference between berbicara, bicara, and ngomong? Why use berbicara here?

All three have the core meaning “to speak / to talk,” but the tone differs:

berbicara – more formal or neutral; fine in writing, news, formal speech.
bicara – a slightly shorter, still neutral form; common in both speech and writing.
ngomong / ngomongin – clearly informal/colloquial, used in casual conversations.

In this sentence, berbicara makes the sentence sound neutral-standard, suitable for written examples. You could say Dia bicara kasar… or Dia ngomong kasar… with the same basic meaning, but the style becomes more informal.

What exactly does kasar mean in berbicara kasar? Is it only “rude”?

In this context, kasar means rude / impolite / harsh in the way someone speaks.
More generally, kasar can also mean rough (texture), crude, or vulgar, depending on context.
So berbicara kasar suggests the person used impolite language (maybe swearing, insulting, or speaking in a very harsh tone), not just being a little blunt.

Why is it di grup chat? Does di mean “in”, “at”, or “on”?

Di is a general preposition for in/at/on when talking about locations, both physical and virtual.
In di grup chat, the “location” is the online group chat space, so di is used like “in the group chat.”
You’d also say di WhatsApp, di Instagram, di Zoom, all with the same di. Context decides whether you translate it as “in,” “on,” or “at” in English.

Is grup chat standard Indonesian? Could I say it another way?

Grup chat is widely used and understood; it’s a loan phrase from English, very common for messaging apps.
More “Indonesian-sounding” alternatives include grup obrolan, grup percakapan, or grup WhatsApp / grup WA if you want to specify the app.
However, in everyday modern Indonesian, grup chat is completely natural.

What does the hyphen in teman-teman do? Why not just teman?

Reduplication (repeating the word) is one common way to show plural in Indonesian.
So:

teman = friend
teman-teman = friends

You don’t have to mark plural; teman can also mean “friends” if it’s clear from context. Using teman-teman just makes the plurality explicit, like saying his friends or the friends as a group.

Why does the sentence say teman-teman instead of mereka?

Both are possible, but they focus slightly differently:

teman-teman diam saja emphasizes that his/her friends (this specific group) stayed silent.
mereka diam saja just says they stayed silent, which could be any group previously mentioned.

Using teman-teman ties the second clause clearly back to his/her friends in the context of the group chat.

Could I drop the second subject and say Dia berbicara kasar di grup chat, dan diam saja?

Yes, you can say …dan diam saja, and it can be understood, but it becomes ambiguous.
It could mean:

He/She spoke rudely … and just kept quiet (same person), or
He/She spoke rudely … and (they) just kept quiet (the others).

By repeating teman-teman, the original sentence makes it very clear that it’s the friends who stayed quiet.

What does diam saja add exactly? Is it the same as just diam?

Diam means to be silent / to stay still / not respond.
Adding saja roughly adds the nuance just / only, often with a slight flavor of passivity or disappointment:

diam = (they) were silent
diam saja = (they) just stayed silent / only stayed silent (did nothing else)

You can often replace diam saja with hanya diam without changing the meaning much; hanya is a bit more “textbook,” saja more colloquial-feeling.

How do we know this sentence is in the past (“spoke rudely”) when there’s no tense marking?

Indonesian verbs do not change form for tense. Berbicara by itself is tenseless.
Time is understood from context or from time words like tadi, kemarin, sekarang, nanti, etc.
Here, if this sentence appears in a narrative about something that already happened, you naturally translate it as “He/She spoke rudely…” in English; in a live commentary, you might translate it as “is speaking rudely…” instead.

Is berbicara kasar the same as berbicara dengan kasar?

They are very close in meaning:

berbicara kasar
berbicara dengan kasar

Both mean “to speak rudely/harshly.”
Dengan literally means “with,” so dengan kasar is more like “in a rude way,” but Indonesian very often just puts the adjective after the verb (as in berbicara kasar) instead of using dengan. The version without dengan is shorter and very natural.

Is the comma before dan necessary: …di grup chat, dan teman-teman diam saja?

In Indonesian, you may use a comma before dan when it connects two independent clauses, each with its own subject and verb, as in this sentence.
So the comma here is acceptable and common:

Dia berbicara kasar di grup chat, dan teman-teman diam saja.

You will also see it without the comma in informal writing. The meaning doesn’t change.

Could we use tapi instead of dan here? Would that change the nuance?

Yes, you could say:

Dia berbicara kasar di grup chat, tapi teman-teman diam saja.

Dan = and, simply adds another fact.
Tapi = but, adds a slight contrast, suggesting something like: even though he was being rude, the friends still stayed silent.
So tapi makes the friends’ silence feel more noteworthy or unexpected.