Learn Hardcore Indonesian: Menurut pelanggan lama, kualitas barang di toko itu bagus. - According to longtime customers, the quality of goods in that shop is good.

Examples: Menurut saya (according to me/in my opinion), Menurut berita (according to the news), Menurut aturan (according to the rules).
If you need a clause after it, use a noun-y structure: Menurut apa yang dia katakan (according to what he said).
Contrast: katanya = “they say/it’s said” (more hearsay), while menurut cites a source or viewpoint.

Does “pelanggan lama” mean “old customers” in age?

No. Lama here means “long-time.” Pelanggan lama = long-time/longstanding customers. For age, use tua: pelanggan yang sudah tua (elderly customers). For former customers, say mantan pelanggan.

Is “pelanggan lama” singular or plural?

Indonesian doesn’t mark plural by default, so it can be either. Add a marker if you need to be explicit:

Singular: seorang pelanggan lama (a long-time customer)
Plural (people): para pelanggan lama (long-time customers as a group)
Quantified: beberapa pelanggan lama, dua orang pelanggan lama, banyak pelanggan lama

Why is it “toko itu” and not “itu toko”?

As a demonstrative determiner, ini/itu usually follows the noun: toko itu = “that store.”
Pre-noun itu can occur when itu acts more like a standalone “that” introducing a clause, e.g., Itu toko favorit saya (“That is my favorite store”). For “that store” inside a noun phrase, prefer toko itu.

Where is the verb “to be”? Why isn’t there “adalah”?

Indonesian uses adjectives as predicates without a copula: kualitas … bagus = “the quality is good.”
Use adalah mainly to equate two nouns: Dia adalah guru (He is a teacher). Don’t say … adalah bagus before an adjective.

Can I say “kualitas barangnya bagus” instead?

Yes, if the owner/source is clear from context. Barangnya = “the goods (its/their goods).”

Kualitas barang di toko itu bagus explicitly points to that store.
Kualitas barangnya bagus is shorter but relies on previous mention of the store/person.

Do I need to say “barang-barang” instead of “barang”?

No. Barang already covers “goods” generically. Reduplication (barang-barang) can emphasize plurality/variety, but here it’s unnecessary and can sound heavy. Kualitas barang is the natural collocation.

What’s the difference between “bagus” and “baik”?

Bagus: good in quality/appearance/usefulness, usually for things. Ex: Film itu bagus, Tasnya bagus.
Baik: good in a moral/functional sense; also “well.” For quality statements, kualitasnya baik is also acceptable, but for physical products bagus is very common.
Rule of thumb here: kualitas barang … bagus/baik both work; bagus is slightly more colloquial for products; baik can sound a bit more formal/neutral.

Why is it “di toko itu”? Could I use “ke/dari/pada”?

di = at/in (location): di toko itu (at that store).
ke = to (direction): ke toko itu (to that store).
dari = from (source): dari toko itu (from that store).
pada is more formal and used for time, recipients, or abstract relations; not for physical location here.

Should there be a “yang” (e.g., “kualitas barang yang di toko itu …”)?

No. Yang introduces a relative clause. You’d use it if you add a verb/explanation:

Good: kualitas barang yang dijual di toko itu bagus (the quality of the goods that are sold at that store is good).
Not good: yang di toko itu without a verb.

Are there other natural ways to express the same idea?

Yes:

Menurut pelanggan lama, barang di toko itu berkualitas tinggi.
Menurut pelanggan lama, mutu barang di toko itu baik.
Menurut para pelanggan lama, barang di toko itu bagus kualitasnya.
Menurut pelanggan lama, toko itu menjual barang berkualitas.

Is “kualitas” the only word for “quality”?

No. Mutu is a near-synonym (often formal/technical). You can also use berkualitas (tinggi) to mean “of (high) quality.”

mutu barang (product quality)
barang berkualitas tinggi (high-quality goods)

Do I need the comma after “Menurut pelanggan lama”?

It’s recommended in writing because the phrase is a fronted adverbial (“According to …”). In casual text you might see it omitted, but the comma improves clarity.

Any spelling pitfalls with “di”?

Yes:

di as a preposition (location) is written separately: di toko.
di- as a passive prefix attaches to verbs: dijual, dibeli.
Your sentence uses the preposition: di toko (with a space).

What’s the difference between “pelanggan lama,” “pelanggan tetap,” and “langganan”?

Pelanggan lama: long-time customers (focus on duration).
Pelanggan tetap: regular/loyal customers (focus on regularity/loyalty).
Langganan: can mean “subscription/regular” or “regular place/person,” e.g., warung langganan saya (my regular stall). As “subscriber,” you might see pelanggan too, depending on context.