Di supermarket besar, saya mengambil troli karena mau membeli banyak barang.

Di is a preposition here, meaning location: in / at.

So di supermarket besar means something like in the big supermarket or at the big supermarket, depending on how natural English would express it.

A useful contrast:

• di supermarket = at/in the supermarket
• ke supermarket = to the supermarket

So:

• Di supermarket besar, saya mengambil troli... = the action happened there
• Saya pergi ke supermarket besar... = I went there

Also, this di is separate from the passive prefix di-:

• di supermarket = preposition, written separately
• dibeli = passive verb, written together

In Indonesian, adjectives usually come after the noun.

So:

• supermarket besar = big supermarket
• barang banyak would sound odd for many items, because banyak behaves a bit differently, but with normal adjectives the pattern is usually noun + adjective

More examples:

• rumah besar = big house
• mobil baru = new car
• toko kecil = small shop

You can also say supermarket yang besar, which feels a bit more explicit: the supermarket that is big / the big supermarket. But supermarket besar is the basic pattern.

The phrase Di supermarket besar has been moved to the front of the sentence to set the scene.

That makes it an introductory phrase, and in writing it is often followed by a comma:

• Di supermarket besar, saya mengambil troli...

Without the comma, the sentence is still understandable:

• Di supermarket besar saya mengambil troli...

The comma just helps readability. It is especially common when the opening phrase is a kind of time/place setting.

Yes, saya could be omitted in some contexts, because Indonesian often leaves out subjects when they are already clear.

But in a standalone sentence, keeping saya makes it clear and complete:

• Saya mengambil troli... = I took a cart...

Saya is a neutral-to-polite word for I.
In more informal speech, someone might say:

• aku
• or just omit the subject if it is obvious

So:

• Saya mengambil troli... = neutral/polite
• Aku ambil troli... = more casual

Mengambil is the meN- form of the root ambil.

• ambil = root / base form
• mengambil = active verb, often more neutral in formal or careful Indonesian

In everyday conversation, Indonesians very often use the shorter form:

• Saya ambil troli...

In more standard or written style, mengambil is very natural:

• Saya mengambil troli...

So both can work, but:

• mengambil = more standard/formal
• ambil = more casual/colloquial

Yes. Literally, mengambil troli means to take/get a trolley/cart.

In this supermarket context, it means taking a shopping cart to use while shopping.

Depending on region and context:

• troli can mean trolley
• in American English, the natural translation is often shopping cart
• in British English, trolley fits well

If you want to be extra specific, Indonesian can also say:

• troli belanja = shopping trolley/cart

But just troli is very normal if the context is already a supermarket.

Karena means because, and mau membeli means wanted/planned/intended to buy.

So the idea is:

• I took a cart because I wanted to buy many items.

If you said karena membeli banyak barang, that would sound more like:

• because of buying many items
• because I was buying many items

That is a different nuance.

Mau adds the idea of intention or plan, which fits the situation well: you grab a cart because you expect to buy a lot.

Mau can mean several related things, including:

• want to
• about to
• going to
• intend to

Here, it expresses intention:

• karena mau membeli banyak barang = because I wanted / was going to buy many items

In everyday Indonesian, mau is extremely common and often more natural than a more formal word like ingin.

Compare:

• mau beli = want to buy / going to buy
• ingin membeli = want to buy, but more formal or deliberate

So in this sentence, mau is very natural.

Both are possible.

• mau beli = very common in speech, casual and natural
• mau membeli = a little more formal or careful

The verb membeli is the meN- form of beli:

• beli = buy
• membeli = to buy

After mau, everyday Indonesian often prefers the shorter base form:

• Saya mau beli banyak barang.

But using membeli is still correct, especially in more standard written Indonesian:

• Saya mau membeli banyak barang.

It could be repeated, but it does not have to be.

Indonesian often omits a subject when it is clearly the same as the previous one. So:

• saya mengambil troli karena mau membeli banyak barang naturally means:
• I took a cart because I wanted to buy many items

If you want to be more explicit, you can say:

• ...karena saya mau membeli banyak barang

Both are correct.
Without saya, the sentence is slightly lighter and very natural.

Because banyak already tells you the noun is plural or numerous.

So:

• banyak barang = many items / lots of things

In Indonesian, nouns do not need a separate plural ending like English -s.

You can also say:

• banyak barang-barang

But that is often felt to be unnecessary or less elegant, because both banyak and reduplication are marking plurality. Standard Indonesian usually prefers:

• banyak barang

Useful comparison:

• barang = item / thing / goods
• barang-barang = items / things
• banyak barang = many items

Indonesian does not use articles the way English does.

So there is no direct equivalent of:

• a
• an
• the

Whether something is definite or indefinite is usually understood from context.

That is why Indonesian can simply say:

• di supermarket besar
• mengambil troli
• banyak barang

If needed, Indonesian can add words for specificity, for example:

• sebuah troli = a trolley/cart
• troli itu = the trolley / that trolley
• supermarket itu = the supermarket / that supermarket

But in your sentence, no article is necessary.

Barang is a very common word meaning:

• thing
• item
• goods

In a shopping context, banyak barang naturally means many items or a lot of groceries/products/things.

It is a broad, useful word. Depending on context, other nouns are also possible:

• produk = products
• belanjaan = shopping, purchased items, groceries
• bahan makanan = food ingredients
• groceries is sometimes borrowed in informal bilingual settings, but that is not standard Indonesian

So banyak barang is normal and easy to understand.

Yes, the sentence is correct and natural, especially in a neutral or written style.

A more everyday spoken version might be:

• Di supermarket besar, saya ambil troli karena mau beli banyak barang.

That version sounds more conversational because:

• ambil is more casual than mengambil
• beli is more casual than membeli

Another very natural variation is:

• Di supermarket besar, saya mengambil troli karena akan membeli banyak barang.

That sounds a bit more formal or planned, because akan is more clearly will / going to.

So your original sentence is good; it just leans slightly toward standard written Indonesian rather than casual speech.