Learn Hardcore Indonesian: Kabar baik itu didengar oleh keluarga saya di ruang tamu. - That good news was heard by my family in the living room.

Itu is a demonstrative that basically means that, but after a noun phrase it often works like a general marker of definiteness, similar to English the or that depending on context.

kabar baik = good news (not specific)
kabar baik itu = that good news / the good news (specific, already known or just mentioned)

So itu shows that the speaker and listener both know which good news is being talked about. Without itu, it would sound more like some good news in general, not a specific piece of news.

Why is didengar used instead of mendengar here?

Didengar is the passive form of mendengar (to hear).

Active: Keluarga saya mendengar kabar baik itu di ruang tamu.
→ My family heard that good news in the living room.
Passive: Kabar baik itu didengar oleh keluarga saya di ruang tamu.
→ That good news was heard by my family in the living room.

In the passive sentence:

kabar baik itu becomes the grammatical subject.
keluarga saya is the agent, introduced by oleh.

Indonesian uses the passive when the speaker wants to highlight the object (here, the good news) rather than the doer (the family). Grammatically, both active and passive versions are correct; it is a matter of focus and style.

Is oleh necessary in this sentence? Can I say Kabar baik itu didengar keluarga saya di ruang tamu?

Yes, you can drop oleh in everyday Indonesian:

Kabar baik itu didengar keluarga saya di ruang tamu.

This is still passive, and keluarga saya is clearly understood as the agent (by my family).

Some notes:

With oleh: more formal or explicit, and very common in writing.
Kabar baik itu didengar oleh keluarga saya...
Without oleh: very common in speech and informal writing.
Kabar baik itu didengar keluarga saya...

You should keep oleh if leaving it out could cause confusion about who did what, but here it is clear, so omitting it is fine.

Could we use terdengar instead of didengar? What is the difference?

You can say:

Kabar baik itu terdengar oleh keluarga saya di ruang tamu.

However, terdengar and didengar are not identical.

didengar (di- passive): focuses on the action done by someone
→ the family heard the news (intentional action).
terdengar (ter- form): focuses on the state of being audible or being heard, often more neutral or unintentional
→ the news could be heard / was audible to the family.

Nuance:

didengar: more about the family as hearers: the family heard that good news.
terdengar: more about the news as a sound: that good news was heard / could be heard by the family.

Both can be correct; use terdengar if you want to emphasise that the news reached their ears or was audible, not necessarily that they actively listened.

What is the difference between kabar baik, berita baik, and kabar gembira?

All can relate to good news, but they have slightly different feels:

kabar baik
- Literally: good news
- Very common, neutral, can be everyday personal news.
- Example: Saya punya kabar baik untuk kamu. (I have good news for you.)
berita baik
- Also good news, but berita often sounds a bit more formal or like information, reports, or news items.
- Natural in more formal contexts, official announcements, etc.
kabar gembira
- Literally: joyful news / delightful news.
- More emotional, emphasising happiness or excitement.
- Often used in written announcements or dramatic speech.

In your sentence, kabar baik itu is natural and sounds like specific good personal or family-related news.

Does keluarga saya mean my family as a single unit, or my family members? How does number work here?

Keluarga saya can mean both, depending on context:

As a unit: my family (seen as one group)
As members: my family members / my relatives

Indonesian verbs do not change for singular or plural, so didengar stays the same whether keluarga saya is thought of as one group or many individuals.

In this sentence, keluarga saya would usually be understood as my family (members) heard the news, without needing to choose strictly between singular or plural.

Can di ruang tamu be moved to another position, like Di ruang tamu, kabar baik itu didengar oleh keluarga saya?

Yes. Word order with time and place phrases is quite flexible. All of these are possible:

Kabar baik itu didengar oleh keluarga saya di ruang tamu.
(neutral: place at the end)
Di ruang tamu, kabar baik itu didengar oleh keluarga saya.
(emphasis on the location)
Kabar baik itu di ruang tamu didengar oleh keluarga saya.
This is grammatically possible but sounds a bit heavier or more literary; everyday speech prefers option 1 or 2.

Moving di ruang tamu to the front emphasises where the event happened. The basic meaning stays the same.

In English we say in the living room, but Indonesian just has di ruang tamu without any word for the. Is that normal?

Yes, that is normal. Indonesian usually does not use an article like the or a.

di ruang tamu can mean in the living room or in a living room, depending on context.
If you need to be specific, you can add itu or ini:
- di ruang tamu itu = in that living room / in the living room (already known)
- di ruang tamu ini = in this living room

In your sentence, di ruang tamu is naturally understood as in the living room of the speaker’s house, without needing any extra word.

How do we know when this action happened? There is no tense marker like was or has in the Indonesian sentence.

Indonesian verbs do not change for tense. Didengar itself does not say when it happened; it only shows passive voice.

Time is expressed through context or time expressions:

Kemarin kabar baik itu didengar oleh keluarga saya di ruang tamu.
(Yesterday that good news was heard by my family in the living room.)
Tadi pagi kabar baik itu didengar...
(This morning that good news was heard...)

Without a time word, kabar baik itu didengar... could be translated as was heard, has been heard, or even is heard, depending on context. English must choose a tense; Indonesian does not.

Is this sentence an example of passive voice type I? Could we also make a passive type II version?

Yes, this is passive voice type I:

Pattern: di-verb + (oleh + agent)
Kabar baik itu didengar oleh keluarga saya di ruang tamu.

A passive type II version is also possible, especially in spoken Indonesian. In type II, you keep the me- verb and move a pronoun (or sometimes a short noun) in front of it as the agent:

Kabar baik itu saya dengar di ruang tamu.

Here:

kabar baik itu is still the topic/focus.
saya before dengar acts as the agent (I heard).
oleh is not used.

If you want to keep keluarga saya explicitly, type II is less natural because the agent is usually a short pronoun, but you might hear:

Kabar baik itu kami dengar di ruang tamu. (We heard that good news in the living room.)

So your original sentence is a clear type I passive; type II is more common with pronoun agents like saya, kami, dia, mereka, and is frequent in everyday speech.