Hindi lang maganda ang pulang damit, kundi bagay rin ito sa itim mong sapatos.

It is a very common pattern meaning not only ... but also ....

So the structure is:

hindi lang + first idea, kundi + second idea

In this sentence, the speaker says that the red dress is not only beautiful, but it also goes well with your black shoes.

Yes. Lang is the shorter, more conversational form of lamang.

So:

• hindi lang = not only
• hindi lamang = the same meaning, but a bit more formal

Both are correct.

Because Filipino often uses predicate-first word order.

Here:

• maganda = the predicate adjective
• ang pulang damit = the topic/subject

So the structure is closer to:

Beautiful is the red dress

Of course, that sounds unnatural in English, so we translate it as The red dress is beautiful.

You could also say:

Ang pulang damit ay hindi lang maganda...

That version is also correct, but the original sentence is very natural in Filipino.

Not exactly. Ang is not simply the English article the.

In Filipino, ang often marks the topic or subject-like part of the sentence.

So in:

ang pulang damit

ang marks pulang damit as the thing being talked about.

Sometimes ang may correspond to the in translation, but its job in Filipino grammar is different.

Because Filipino uses a linker between a modifier and the word it modifies.

The adjective pula ends in a vowel, so it takes the linker -ng:

• pula
  • -ng
    • damit
• pulang damit

This is the normal way to say red dress.

Yes, you can.

Both are grammatical:

• pulang damit
• damit na pula

But pulang damit is the more compact and very common adjective-before-noun pattern.

damit na pula can sound a little more descriptive or contrastive depending on context, like when you are distinguishing it from clothes of other colors.

Kundi is commonly used after a negative idea, especially in patterns like:

• hindi ... kundi ...
• hindi lang ... kundi ...

It means something like but rather, but instead, or in this pattern, but also.

So kundi fits very naturally after hindi lang.

Pero just means but, and it does not match this pattern as neatly. A sentence with pero could still be understandable in some contexts, but kundi is the better choice here.

Here, bagay means to suit, to match, or to go well with.

So:

• bagay ito sa sapatos = it goes well with the shoes
• bagay sa iyo = it suits you

Be careful, because bagay can also be a noun meaning thing. In this sentence, it is clearly being used as a predicate meaning matches/suits.

Ito refers back to ang pulang damit.

Instead of repeating ang pulang damit, Filipino often uses a pronoun:

• bagay rin ito ... = it also matches ...

It comes after bagay rin because predicate-first order is normal in Filipino.

Also, pronouns like ito do not need ang before them here.

So:

• bagay rin ito sa ... = natural
• ito rin ay bagay sa ... = also correct, but more formal or more marked

Rin and din are the same word. They both mean also / too.

Which form you use usually depends on the sound before it:

• after a vowel, w, or y: rin
• after most other consonants: din

Since bagay ends in y, rin is the preferred form:

• bagay rin

Because the pattern is:

bagay sa + thing/person

So sa introduces the thing that something matches or suits.

Examples:

• Bagay ito sa iyo. = It suits you.
• Bagay ito sa sapatos. = It goes well with the shoes.

In your sentence:

bagay rin ito sa itim mong sapatos

means it matches your black shoes.

Mong is mo combined with a linker.

In constructions like this, when mo comes before a noun as part of a modifying phrase, it often appears as mong.

So:

• itim mong sapatos = your black shoes

This is a very common pattern, similar to:

• maganda mong damit = your beautiful dress
• bago mong libro = your new book

It is a compact noun phrase meaning your black shoes.

You can think of it as:

• itim = black
• mong = your, in linked form
• sapatos = shoes/shoe

So the whole phrase means your black shoes.

A closely related version is:

itim na sapatos mo

This also means your black shoes. Both are correct. The mong version is just a very natural, compact way to build the phrase.

Because Filipino nouns usually do not change form for singular and plural, and speakers do not always add mga unless they want to make plurality explicit.

So sapatos can be understood from context.

In this sentence, it is natural to understand sapatos as shoes or a pair of shoes, even without mga.

If you wanted to mark plural more clearly, you could say mga sapatos, but it is not necessary here.