Learn Hardcore Indonesian: Belakangan, pacar dia sering ikut makan siang bersama kami. - Lately, her/his partner often joins us for lunch.

Belakangan here means lately/recently.
Belakangan ini and akhir-akhir ini both mean these days/lately and often sound a bit more natural in everyday speech when you mean an ongoing trend.
Baru-baru ini means recently but usually points to one or a few specific recent events, not an ongoing pattern.
In your sentence, because of sering (often), belakangan (ini) and akhir-akhir ini are the best fits; baru-baru ini would feel off.

Examples:

Belakangan (ini), pacar dia sering ikut makan siang... (ongoing trend)
Baru-baru ini, pacar dia ikut makan siang... (suggests a recent occurrence, not necessarily frequent)

Can belakangan also mean later on (in the future/past), and if so, why doesn’t it here?

Yes. Belakangan can mean later (on) or afterwards in other contexts:

Nanti belakangan saja. (Let’s do it later.)
Belakangan ketahuan bahwa… (It was later found out that…)

In your sentence, the comma and placement at the start, plus the collocation with sering, make the intended meaning lately (a time frame in the recent past), not later.

Do I need the comma after Belakangan?

It’s recommended but not strictly required. A comma after a sentence-initial time adverb like Belakangan helps readability:

Belakangan, pacar dia... ✔ (clearer)
Belakangan pacar dia... ✔ (still acceptable)

What’s the difference between pacar dia and pacarnya? Are both correct?

Both are correct and both can mean his/her boyfriend/girlfriend:

Pacar dia explicitly uses the pronoun dia (he/she).
Pacarnya uses the clitic -nya, which usually refers to a third person (his/her/their) or can mark definiteness (the).

Nuance:

Pacarnya is a bit tighter/more natural in many contexts.
Pacar dia can feel a touch more explicit or contrastive (emphasizing whose partner it is).

Can I say pacarnya dia?

Yes. Pacarnya dia is common in speech. It double-marks possession (with -nya and dia), which can sound redundant on paper but is natural in conversation, often used to clarify or emphasize the possessor:

Belakangan, pacarnya dia sering ikut... ✔

Does dia mean he or she? How would I specify gender?

Dia is gender-neutral (he/she). To specify:

Pacar laki-lakinya / cowoknya (boyfriend)
Pacar perempuannya / ceweknya (girlfriend) Register note: cowok/cewek are casual/colloquial; laki-laki/perempuan are neutral/formal.

Could I replace dia with ia?

Generally no in this slot. Ia is a more formal subject pronoun and doesn’t work well as a possessor after a noun. So:

Pacar dia ✔
Pacarnya ✔
Pacar ia ✘ (unnatural) If you want a more formal feel, use -nya: pacarnya.

Is pacar gendered or does it imply exclusivity? Could it mean spouse?

Pacar is gender-neutral and usually implies an exclusive boyfriend/girlfriend (dating partner).
For a spouse, use suami (husband) / istri (wife).
Pasangan means partner (neutral, can be romantic or spousal, and is often more formal).

What does ikut add? Could I drop it?

Ikut means to join in / go along. It adds the sense that there’s an existing plan/activity and the person joins it.

With ikut: sering ikut makan siang = often joins (us) for lunch.
Without ikut: sering makan siang bersama kami = often has lunch with us (neutral statement, less “joining” nuance).

Alternatives:

Bergabung (dengan kami) untuk makan siang (more formal/literal “to join”)
Menemani kami makan siang (to accompany us for lunch; nuance: keeps us company)

Where should sering go? Can I say ikut sering makan siang?

Put sering before the main verb:

Pacar dia sering ikut makan siang... ✔
Pacar dia ikut sering makan siang... ✘ (feels wrong)
Sering can also go near the front for emphasis: Belakangan, pacar dia sering ikut... ✔

Variants:

Sering kali (quite frequent)
Kerap / acap (kali) (more formal/poetic)

Is makan siang a verb or a noun here?

It functions as a verb phrase meaning to have lunch. Indonesian often uses makan + time-of-day noun this way:

makan pagi (have breakfast) = more common is sarapan
makan siang (have lunch)
makan malam (have dinner)

It can also be a noun (lunch) depending on context, but here it’s verbal.

What’s the difference between bersama kami and dengan kami? And what about casual options like bareng or sama?

All can mean with us, but register and feel differ:

Bersama kami: neutral to slightly formal, “together with us.”
Dengan kami: neutral, very common, “with us.”
Bareng kami/kita: casual/colloquial, “together with us.”
Sama kami/kita: very colloquial, “with us.”

Choose based on formality and setting.

Why kami and not kita?

Kami = we/us (excluding the listener).
Kita = we/us (including the listener).

If you’re talking to someone who is not part of the lunch group, kami is right. If you’re talking to someone who is part of the group, use kita:

Belakangan, pacar dia sering ikut makan siang bersama kita. (includes the listener)

Can I move belakangan to the end?

Yes:

Pacar dia sering ikut makan siang bersama kami belakangan. ✔

This is fine and common in speech. Putting belakangan at the start slightly foregrounds the time frame.

Is the sentence formal or informal? How could I adjust the register?

The original is neutral. Adjustments:

More formal: Akhir-akhir ini, pacarnya sering ikut makan siang bersama kami.
Casual: Belakangan, pacarnya sering ikut makan siang bareng kita.
Very casual slang: Belakangan, doi sering makan siang bareng kita. (using doi for he/she; note strong informality)

Is turut a good replacement for ikut here?

No. Turut is used for polite concurrence or sympathy (e.g., turut berduka cita) and doesn’t fit casual “join us for lunch.” If you want a formal alternative, use ikut serta or bergabung:

sering ikut serta makan siang (very formal)
sering bergabung untuk makan siang (formal/literal)

Can I topicalize pacar dia to emphasize it?

Yes:

Pacar dia, belakangan sering ikut makan siang bersama kami.
Pacar dia belakangan sering ikut makan siang bersama kami.

This places emphasis on pacar dia (his/her partner).

Any small wording tweaks that sound very natural?

These are all natural, with slight nuance shifts:

Belakangan ini, pacarnya sering ikut makan siang bareng kami. (very common)
Akhir-akhir ini, pacarnya sering makan siang dengan kami. (no “join” nuance)
Belakangan, pacarnya jadi sering ikut makan siang dengan kami. (jadi adds “has started to”/“now tends to”)