Learn Hardcore Indonesian: Sekarang mereka masih tunangan, tetapi cincin sudah disiapkan oleh keluarga pengantin perempuan. - Right now they are still engaged, but the rings have already been prepared by the bride’s family.

You don’t need adalah (is/are) between mereka and tunangan.
You also don’t need sedang because being engaged is a state, not an action in progress.

So mereka masih tunangan = they are still (in the state of being) engaged.

What is the difference between tunangan and bertunangan?

Both are related to being engaged, but they’re used differently:

tunangan (noun/adjective-like):
- Dia tunangan saya. = He/She is my fiancé(e).
- Mereka masih tunangan. = They are still engaged.
bertunangan (intransitive verb to be engaged):
- Mereka sudah bertunangan. = They are already engaged / They have become engaged.
- Kami akan bertunangan bulan depan. = We will get engaged next month.

In your sentence, mereka masih tunangan emphasizes their current status.
Mereka masih bertunangan is also possible and natural; it sounds a bit more like describing the state as the result of an event (the engagement).

Why is it cincin sudah disiapkan and not something like keluarga sudah menyiapkan cincin?

Both are correct; they just use different voices:

Passive (used in the sentence):
- Cincin sudah disiapkan oleh keluarga pengantin perempuan.
  Literally: The ring has already been prepared by the bride’s family.
  Focus is on cincin (the ring).
Active:
- Keluarga pengantin perempuan sudah menyiapkan cincin.
  Literally: The bride’s family has already prepared the ring.
  Focus is on keluarga pengantin perempuan (the bride’s family).

Indonesian uses passive a lot, especially in narrative or written style, when you want to highlight the object (cincin) rather than the doer.

How is disiapkan formed, and what is the difference between siap, menyiapkan, and disiapkan?

Morphology:

siap = ready (adjective)
- Cincin sudah siap. = The ring is already ready.
menyiapkan = to prepare (active verb: to make something ready)
- Mereka menyiapkan cincin. = They prepare the ring.
disiapkan = to be prepared (passive verb)
- Cincin disiapkan mereka. = The ring is prepared by them.

Formation of disiapkan:
di- (passive prefix) + siap (root) + -kan (causative / object-focus suffix)
→ disiapkan = to be made ready / to be prepared (by someone).

Is oleh necessary in disiapkan oleh keluarga pengantin perempuan? Can it be omitted?

In everyday Indonesian, oleh is often omitted when the agent is clear:

Cincin sudah disiapkan keluarga pengantin perempuan. (very natural)
Cincin sudah disiapkan oleh keluarga pengantin perempuan. (slightly more formal / explicit)

So oleh is not grammatically required here; it just makes the passive structure a bit more formal and clearly marks the agent.

What exactly does keluarga pengantin perempuan mean, and how is it structured?

keluarga pengantin perempuan literally = family of the bride.

Structure:

keluarga = family
pengantin perempuan = bride (literally female wedding person)

So pengantin perempuan is a unit meaning bride.
Then keluarga pengantin perempuan is a noun followed by another noun phrase indicating possession/association, similar to keluarga Andi = Andi’s family.

So it does not mean female family of the bridegroom; it clearly refers to the bride’s family.

Are there other ways to say pengantin perempuan?

Yes, some common alternatives:

mempelai perempuan – very common in wedding contexts, a bit formal
pengantin wanita – also widely used, slightly less formal than mempelai perempuan
mempelai wanita – similar to pengantin wanita

All of these mean bride. In everyday speech, pengantin perempuan and pengantin wanita are both natural and easily understood.

What is the function of sudah in cincin sudah disiapkan? Is it just already?

sudah here mainly shows that the action is completed:

cincin disiapkan = the ring is prepared (no time/completion information)
cincin sudah disiapkan = the ring has already been prepared / is already prepared

Nuances of sudah:

Completion: the preparation is finished.
Sometimes contrast: suggests it might be earlier than expected, especially when combined with masih in the other clause.

You can think of it as roughly like English already or present perfect has been:

sudah ≈ already / has (been) + past participle.

What is the contrast created by masih in the first clause and sudah in the second?

The sentence:

Sekarang mereka masih tunangan, tetapi cincin sudah disiapkan oleh keluarga pengantin perempuan.

The contrast:

masih = still (the state continues: they are not yet married)
sudah = already (something is already done in preparation for marriage)

So it implies:
They haven’t married yet (still engaged), but preparations are already advanced (the ring is ready). This creates a feeling of progress toward marriage despite their current status.

Could you rewrite the sentence in a more informal, spoken style?

A natural informal version might be:

Sekarang mereka masih tunangan, tapi cincinnya sudah disiapin keluarga pengantin cewek.

Changes:

tetapi → tapi (colloquial)
cincin → cincinnya (often used to refer to the ring)
disiapkan → disiapin (colloquial pronunciation)
pengantin perempuan → pengantin cewek (very casual; cewek = girl, girlfriend)

For polite but less formal speech, you could do:

Sekarang mereka masih tunangan, tapi cincin sudah disiapkan keluarga pengantin perempuan.

Why is there no word for the or a before cincin?

Indonesian has no articles like English a/an/the. Nouns like cincin are bare:

cincin can mean a ring, the ring, or rings depending on context.

In this sentence, context tells us it’s likely the (engagement) ring:

cincin sudah disiapkan → the ring has already been prepared.

If you need to be explicit, you use other words:

sebuah cincin = a ring (one ring, non-specific)
cincin itu = that ring / the ring (specific)

Can sekarang be moved to the middle or end of the sentence?

Yes, but the most natural position is at the beginning. Possible variations:

Sekarang mereka masih tunangan, tetapi cincin sudah disiapkan… (most natural)
Mereka sekarang masih tunangan, tetapi cincin sudah disiapkan… (okay; stresses mereka)
Mereka masih tunangan sekarang, tetapi cincin sudah disiapkan… (okay; mild emphasis on now as the time frame)

All are grammatically fine; placing sekarang at the start clearly sets the time frame for the whole statement.

Is there any difference between tetapi and tapi here?

They both mean but, but differ in register:

tetapi – more formal, common in writing, formal speech, and textbooks.
tapi – informal/colloquial, used in everyday conversation.

In your sentence:

… tetapi cincin sudah disiapkan … (neutral–formal)
… tapi cincin sudah disiapkan … (natural in spoken Indonesian)

Meaning is the same; only the level of formality changes.

Could the subject mereka be omitted in Sekarang mereka masih tunangan?

Yes, if the subject is clear from context, Indonesian often drops it:

Sekarang masih tunangan, tetapi cincin sudah disiapkan…

This would be understood as Right now (they) are still engaged, but the ring has already been prepared…, assuming the people you are talking about are known in the conversation.

Including mereka is slightly clearer, but not always necessary.