Saya isi borang aduan untuk bayaran balik.

Both are possible, but they differ in style.

• mengisi is the more standard/formal verb form.
• isi without the prefix is very common in spoken Malay and informal writing.

So:

• Saya mengisi borang aduan untuk bayaran balik. = more formal/standard
• Saya isi borang aduan untuk bayaran balik. = more conversational

A native speaker would understand both.

Here, isi means fill in or fill out.

So isi borang means:

• fill in a form
• fill out a form

It does not mean contain here, even though isi can also mean contents in other contexts.

Examples:

• Sila isi borang ini. = Please fill in this form.
• Apa isi kotak itu? = What are the contents of that box?

So the meaning depends on context.

Malay normally does not use articles like a, an, or the.

So borang aduan can mean:

• a complaint form
• the complaint form
• sometimes just complaint form

The exact meaning depends on context.

This is very normal in Malay:

• Saya beli buku. = I bought a book / the book
• Dia buka pintu. = He/She opened the door / a door

In Malay, it is very common to put nouns together, where the first noun is the main thing and the second noun describes its type or function.

So:

• borang = form
• aduan = complaint

Therefore:

• borang aduan = complaint form

This is similar to other Malay noun combinations:

• kad kredit = credit card
• rumah sakit (in Indonesian) / hospital in Malay standard usage
• surat khabar = newspaper
• meja makan = dining table

A useful pattern is:

• main noun + describing noun

So borang aduan literally feels like form complaint, but in natural English we say complaint form.

aduan is a noun meaning complaint.

mengadu is a verb meaning to complain.

Compare:

• Saya buat aduan. = I make a complaint.
• Saya mengadu kepada pengurus. = I complain to the manager.

So in borang aduan, the noun aduan is being used:

• borang aduan = complaint form

Here, untuk means for.

So:

• untuk bayaran balik = for a refund

It shows purpose or intended use.

Examples:

• Ini untuk awak. = This is for you.
• Saya datang untuk belajar. = I came to study.
• Borang ini untuk permohonan visa. = This form is for a visa application.

In your sentence, it tells us what the complaint form is related to: a refund.

bayaran balik means refund.

It is made up of:

• bayaran = payment
• balik = back / return

So the idea is literally something like payment back.

This is a common Malay way to express refund.

Examples:

• Saya mahu bayaran balik. = I want a refund.
• Permohonan bayaran balik = refund application
• Polisi bayaran balik = refund policy

Because Malay has fixed or conventional expressions, and bayaran balik is the natural phrase for refund.

Even though balik means back, you cannot always rearrange Malay words freely based on English logic.

So:

• bayaran balik = correct/natural
• balik bayaran = unnatural in this meaning

This is something you often have to learn as a set phrase.

Malay verbs usually do not change form for tense the way English verbs do.

So Saya isi borang aduan untuk bayaran balik could mean:

• I fill in a complaint form for a refund
• I am filling in a complaint form for a refund
• I filled in a complaint form for a refund

The exact time usually comes from context or extra time words.

For example:

• Sekarang saya isi borang aduan... = Now I am filling in...
• Semalam saya isi borang aduan... = Yesterday I filled in...
• Biasanya saya isi borang aduan... = Usually I fill in...

No. Saya is the most neutral and polite word for I.

Other possibilities exist, but they depend on tone and relationship:

• saya = polite, neutral, standard
• aku = informal, closer, more casual
• kami/kita = we, not I

So:

• Saya isi borang... = neutral/polite
• Aku isi borang... = casual, maybe with friends

For learners, saya is the safest choice in most situations.

It is understandable and fairly natural, especially in casual speech. But depending on context, a native speaker might choose a slightly different version.

More standard/formal:

• Saya mengisi borang aduan untuk bayaran balik.

If the meaning is specifically a refund claim form, another phrasing might be:

• Saya mengisi borang tuntutan bayaran balik.

Why?

• aduan = complaint
• tuntutan = claim

So if the form is for making a formal refund claim rather than just submitting a complaint, tuntutan may fit better.

Still, your original sentence is understandable and grammatically workable.

The basic word order is:

• Saya = subject
• isi = verb
• borang aduan = object
• untuk bayaran balik = phrase showing purpose/function

So the structure is:

Subject + Verb + Object + Extra information

This is very common in Malay and often matches English word order quite closely.

Breakdown:

• Saya = I
• isi = fill in
• borang aduan = complaint form
• untuk bayaran balik = for a refund

So it is a straightforward sentence pattern for learners.

In practice, it is most naturally understood as describing the form's purpose:

• a complaint form for a refund

But because Malay can be structurally flexible, a listener may also loosely connect it with the whole action:

• I fill in a complaint form in order to get a refund

In real use, context usually makes this clear. If you want to be more precise, you can rephrase:

• Saya isi borang aduan bagi mendapatkan bayaran balik.
  = I fill in a complaint form to obtain a refund.

That version makes the purpose of the action clearer.