Learn Hardcore Malay: Dompet saya penuh dengan syiling kecil selepas saya bayar di pasar raya. - My wallet is full of small coins after I pay at the supermarket.

You cannot say saya dompet for my wallet; that would be ungrammatical.
Another (more casual) way to say my wallet is dompet aku (informal I / me / my).

What exactly does penuh dengan mean here? Could I just say penuh?

penuh = full
penuh dengan X = full of X

In your sentence:

Dompet saya penuh dengan syiling kecil
= My wallet is full of small coins / change.

You can say just:

Dompet saya penuh. = My wallet is full.

That is grammatically fine, but it doesn’t say what it is full of.
Penuh dengan syiling kecil makes it explicit that it is full of small coins.

Could I say Dompet saya dipenuhi dengan syiling kecil instead? What’s the difference?

Yes, you can say:

Dompet saya dipenuhi dengan syiling kecil.

Differences in nuance:

penuh dengan is simpler and more common in everyday speech.
dipenuhi dengan (passive form) sounds a bit more formal or descriptive, like is filled with / is filled up with.

Meaning-wise, both are fine. For normal conversation, penuh dengan is more natural.

Does syiling kecil literally mean small coins or does it mean small change?

Literally:

syiling = coin
kecil = small

So syiling kecil = small coins.

In context, it is usually understood as small-value coins (like 5, 10, 20 sen) – basically small change.

Other common ways to talk about change:

duit syiling = coins (more general)
duit kecil = small change / small denominations (can include small notes too)

So the sentence implies: My wallet is full of small coins / small change.

Is syiling singular or plural? Why don’t we write syiling-syiling for coins?

Malay usually does not mark plural with -s like English.

syiling can mean coin or coins, depending on context.
Reduplication (syiling-syiling) is possible but only when you really want to emphasize plurality or variety, and it can sound a bit odd here.

In penuh dengan syiling kecil, the idea of many coins is already clear from penuh (full), so you don’t need syiling-syiling.

Why is it syiling kecil, not kecil syiling?

In Malay, adjectives usually come after the noun:

syiling kecil = small coin(s)
rumah besar = big house
kereta baru = new car

So the pattern is:

noun + adjective

Kecil syiling would be wrong in this sentence. It might only appear in special poetic or inverted structures, not in normal speech.

In selepas saya bayar di pasar raya, why is it bayar and not membayar?

Both are actually possible:

selepas saya bayar di pasar raya
selepas saya membayar di pasar raya

Differences:

bayar is the root verb, very common in spoken Malay and fully acceptable in standard Malay.
membayar is the meN- form, a bit more formal or written-style.

With selepas (after), Malay often uses either:

the bare verb (selepas saya makan), or
the meN- verb (selepas saya memandu).

So your sentence with bayar is natural and normal.
Membayar would sound slightly more formal but is also correct.

The action is in the past. Why doesn’t Malay mark past tense, like sudah bayar or use a past verb form?

Malay verbs do not change form for tense (no -ed, no past form).
Time is usually clear from context or from time words.

Your sentence:

Dompet saya penuh dengan syiling kecil selepas saya bayar di pasar raya.

Context and selepas (after) show that the paying happened before the wallet became full.

If you want to make the past more explicit, you can add time markers:

Dompet saya penuh dengan syiling kecil selepas saya tadi bayar di pasar raya.
Dompet saya penuh dengan syiling kecil selepas saya sudah bayar di pasar raya.

But the original is already correct and natural without a special past form.

What is the difference between selepas and lepas? Could I use lepas instead?

selepas = after (more standard / formal)
lepas = a shortened form, very common in spoken Malay

You can say:

Dompet saya penuh dengan syiling kecil lepas saya bayar di pasar raya.

This is natural in everyday conversation.
In writing or more formal contexts, selepas is usually preferred.

What does di pasar raya mean, and how is di used?

di is a preposition meaning at / in / on (location).
pasar raya = supermarket.

So:

di pasar raya = at the supermarket.

Important spelling rule: di as a preposition is written separately:

di pasar raya, di rumah, di sekolah.

(Only when di is a prefix in verbs like dibeli, ditulis is it written together.)

Is there a difference between pasar raya, pasaraya, and using an English word like supermarket?

pasar raya (two words) is the standard Malay form for supermarket.
pasaraya (one word) is a common variant in signs / branding, but pasar raya is more correct in formal writing.
Many people also just say supermarket in casual speech, especially in urban areas:
- bayar di supermarket (very common in speech)
- bayar di pasar raya (more standard Malay)

All three are usually understood.

Could I put the selepas clause at the beginning, like in English: After I paid at the supermarket, my wallet…?

Yes. Both word orders are possible in Malay:

Original style (main clause first):
- Dompet saya penuh dengan syiling kecil selepas saya bayar di pasar raya.
Time clause first:
- Selepas saya bayar di pasar raya, dompet saya penuh dengan syiling kecil.

Both are grammatical and natural.
Putting selepas… at the beginning sounds slightly more narrative, like telling a story.

How would this sentence sound in very casual spoken Malaysian Malay?

A common casual version might be:

Lepas bayar kat supermarket, dompet aku penuh dengan syiling kecil.

Changes:

Selepas → lepas (casual)
di → kat (colloquial at)
pasar raya → supermarket (English loan, very common)
saya → aku (informal I / me / my)

Your original sentence is standard and correct; the casual version is what you’d often hear among friends.