Learn Hardcore Malay: Beg galas itu ringan walaupun saya isi dengan banyak buku. - That backpack is light even though I fill it with many books.

Malay simply puts the adjective after the noun phrase to state what something is like. You only use something like ialah/adalah in more formal, specific structures (often before a noun, not an adjective):

Masalahnya ialah beg ini terlalu berat.
The problem is that this bag is too heavy.

But with adjectives, the linking “is” is normally omitted.

What exactly does beg galas mean? Is galas a noun or a verb here?

Beg galas is a compound noun meaning backpack (a bag you carry on your shoulders).

beg – bag
galas – to sling/carry over the shoulder

Literally, it’s like “sling bag”, but in usage it normally refers to a backpack. On its own, galas is a verb:

Saya galas beg itu. – I sling/carry that bag (on my shoulder).

In beg galas, it has become part of a set noun phrase, much like “backpack” is made of “back” + “pack” in English.

What does itu do in beg galas itu? Is it “that” or “the”?

Itu can mean both “that” (demonstrative) and a kind of definite marker like “the”, depending on context.

beg galas itu
can mean:
- that backpack (specific, maybe pointed at or already known), or
- the backpack (definite, known from context)

Position matters: itu comes after the noun phrase it refers to:

beg itu – that / the bag
budak itu – that / the kid
rumah besar itu – that big house / the big house

In your sentence, it’s most naturally understood as “that backpack” or “the backpack”, referring to a particular one.

What does walaupun mean, and how is it used in this sentence?

Walaupun means “although / even though” and introduces a contrast clause.

Your sentence has two parts:

Beg galas itu ringan – That backpack is light.
walaupun saya isi dengan banyak buku – although I fill (it) with many books.

So the structure is:

[Main clause] + walaupun + [contrasting clause]

In English we normally put “although” before the clause:

The backpack is light even though I fill it with many books.

In Malay, you can also move the walaupun-clause to the front:

Walaupun saya isi dengan banyak buku, beg galas itu ringan.
Although I fill it with many books, the backpack is light.

Both word orders are correct.

Can I replace walaupun with walau or meskipun? Are they different?

They’re very close in meaning; all can mean “although / even though”, but there are style and register differences:

walaupun – very common, neutral; works in speech and writing.
walau – a bit shorter, often more casual; common in speech and informal writing.
meskipun – a bit more formal or literary, but still common.

You can say:

Beg galas itu ringan walau saya isi dengan banyak buku.
Beg galas itu ringan meskipun saya isi dengan banyak buku.

All three are understandable and acceptable. Walaupun is a safe, standard choice.

Why is it saya isi and not saya mengisi? Is leaving out meN- (the prefix) correct?

The base verb is isi (“to fill”), and the standard meN- form is mengisi.

saya mengisi – I fill / am filling (standard / neutral)
saya isi – I fill / am filling (colloquial / conversational)

In conversational Malay, speakers often drop the meN- prefix, especially when the meaning is obvious and the verb comes directly after the subject pronoun:

Saya makan. (standard) → often just saya makan. (same)
Saya mengisi beg itu. → conversationally can become Saya isi beg itu.

So:

Beg galas itu ringan walaupun saya mengisi (beg itu) dengan banyak buku.
More formal/standard.
Beg galas itu ringan walaupun saya isi dengan banyak buku.
More informal/natural in everyday speech.

Both are grammatically fine; the second just sounds more spoken.

Where is the object of isi? What exactly is being filled?

The direct object (the thing being filled) is not stated, but it is understood from context: it’s the beg galas itu (that backpack).

If you make it explicit:

walaupun saya mengisi beg itu dengan banyak buku
although I fill that bag with many books

In casual speech, Malay often omits obvious objects when they’re already mentioned or clear:

Saya dah beli. – I already bought (it).
Dah makan? – (Have you) eaten?

Here, mentioning the bag again would be slightly repetitive, so it’s left out and understood:

…walaupun saya isi (beg itu) dengan banyak buku.

Why do we use dengan in isi dengan banyak buku? Could you say it another way?

Dengan here functions like English “with” in “fill it with many books”.

Pattern:

isi / mengisi X dengan Y – to fill X with Y

Even though X (the bag) is omitted, the pattern is still recognized:

(Saya mengisi beg itu) dengan banyak buku.

You could also express the same idea with different structures:

Saya mengisi banyak buku ke dalam beg itu. – I put / filled many books into that bag.
Saya masukkan banyak buku ke dalam beg itu. – I put many books into that bag.

But isi … dengan … is a very natural, compact way to say “fill (something) with (something)”.

Does banyak buku need a classifier like buah? Is banyak buku alone okay?

Banyak buku is completely natural and correct.

banyak – many / a lot of
buku – books

In Malay, classifiers (penjodoh bilangan) are important mainly when you have a number:

tiga buah buku – three books
dua helai kertas – two sheets of paper

With banyak, you can:

Just say banyak buku – many books (very common), or
Add a classifier banyak buah buku – also correct, slightly more formal or emphatic.

So your original banyak buku is both natural and standard.

Is this sentence talking about the past, present, or future? How does Malay show tense here?

Malay verbs do not change form for tense (no -ed, -s, etc.). Isi here is tense-neutral.

The time frame is understood from context or from extra time words if you add them:

Beg galas itu ringan walaupun tadi saya isi dengan banyak buku.
That backpack was light even though I filled it with many books just now. (past)
Beg galas itu ringan walaupun saya sedang isi dengan banyak buku.
…even though I am filling it with many books. (present / in progress)
Beg galas itu akan ringan walaupun nanti saya isi dengan banyak buku.
…even though I will fill it with many books. (future)

Without any time markers, Beg galas itu ringan walaupun saya isi dengan banyak buku most naturally feels like present/general:
“The backpack is (still) light even though I (do) fill it with many books.”

Is saya the only way to say “I” here? Could you use aku?

Saya is the neutral and polite first-person pronoun, safe in almost all contexts:

with strangers
in semi-formal situations
in writing

You could use aku instead:

Beg galas itu ringan walaupun aku isi dengan banyak buku.

Difference:

aku – more informal, intimate, or casual; used with friends, close family, or in certain dialects.
saya – more polite / neutral.

Grammatically both work; you just choose based on relationship and formality. In a textbook or general example, saya is the best default.

Can the order of the clauses be reversed, like in English “Although I fill it with many books, the backpack is light”?

Yes. Both orders are acceptable in Malay.

Original:

Beg galas itu ringan walaupun saya isi dengan banyak buku.

Reversed:

Walaupun saya isi dengan banyak buku, beg galas itu ringan.

The meaning stays the same; only the emphasis shifts slightly:

Starting with walaupun… puts more focus on the contrast condition.
Ending with walaupun… feels a bit more like adding an afterthought contrast.

Both are natural.

Is there a more formal or “complete” version of this sentence?

Yes. You can make it more formal and explicit by:

using mengisi instead of isi
stating the object (beg itu) or using a pronoun (mengisinya)
optionally adding tetap (“still / remains”) for emphasis

Examples:

Beg galas itu tetap ringan walaupun saya mengisi beg itu dengan banyak buku.
Beg galas itu tetap ringan walaupun saya mengisinya dengan banyak buku.

These sound more carefully written or formal, whereas your original sentence sounds more natural in everyday speech.