Mbak di loket membantu saya mengisi formulir.

Mbak is a polite way to refer to a young woman or an older girl/woman, especially in Javanese-influenced Indonesian usage. In this sentence, it works like the lady / the young woman.

So Mbak di loket means something like the lady at the counter.

It is very common in everyday Indonesian to use words like Mbak, Mas, Pak, and Bu instead of a person’s name.

In Indonesian, words like Mbak, Pak, Bu, and similar titles can function directly as nouns or noun phrases. So Mbak itself can be the subject.

You do not need to add a separate word for she.
So:

• Mbak membantu saya = The lady helped me
• not necessarily She, the lady, helped me

Indonesian often uses titles and kinship terms this way.

Di loket means at the counter, at the service window, or at the ticket/payment desk, depending on context.

• di = at / in / on
• loket = counter, booth, service window

So Mbak di loket literally means the lady at the counter.

Most naturally, di loket describes Mbak:
Mbak di loket = the lady at the counter.

That is the most likely reading.

In practice, Indonesian can sometimes leave this kind of attachment a little flexible, but here the normal interpretation is that it identifies which Mbak we mean.

Because this di is a preposition meaning at / in / on, so it is written separately:

• di loket = at the counter
• di kantor = at the office

This is different from the prefix di-, which forms the passive and is attached to the verb:

• diisi = filled in
• dibantu = helped

So:

• di loket → separate
• diisi → attached

Membantu means to help.

Its base is bantu = help.
With the active verb prefix meN-, it becomes membantu.

This prefix is very common in Indonesian and often forms active verbs:

• bantu → membantu = to help
• isi → mengisi = to fill in
• baca → membaca = to read

You do not need to translate the prefix separately every time; just learn the full verb as the normal active form.

Saya can mean both I and me, depending on its role in the sentence.

Here, saya comes after the verb membantu, so it is the object:

• Mbak ... membantu saya = The lady ... helped me

Indonesian pronouns usually do not change form the way English pronouns do.

Compare:

• Saya mengisi formulir. = I fill out the form.
• Mbak membantu saya. = The lady helps me.

Same word, different grammatical role.

This is a very common Indonesian pattern. The sentence literally looks like:

The lady at the counter helped me fill out the form.

So:

• membantu = helped
• mengisi formulir = fill out the form

After membantu, Indonesian can directly follow with another verb. English often does something similar with help someone do something.

So:

• membantu saya mengisi formulir
• = helped me fill out the form

Because Indonesian does not need it here.

In English, we say:

• helped me fill out the form or
• helped me to fill out the form

In Indonesian, after membantu, the next verb can come directly:

• membantu saya mengisi formulir

No extra word like to is required.

Mengisi means to fill, to fill in, or to fill out, depending on context.

Here, because the object is formulir, the natural translation is fill out or fill in a form.

Its base is isi = contents / fill / content.
With the active prefix meN-, it becomes mengisi.

Both are possible, but they mean slightly different things.

• mengisi formulir = fill out a form / the form
• mengisi formulirnya = fill out the form / his-her-their form / the form in question

In this sentence, formulir is already clear enough, so -nya is not necessary.

Indonesian often leaves things less explicitly marked than English if the meaning is already clear from context.

No, it is a borrowed word, ultimately related to form/formulaire/formulary type words found in European languages. In Indonesian, formulir simply means form.

It is a standard and very common word.

The basic structure is:

Mbak di loket | membantu | saya | mengisi formulir

That is:

• subject: Mbak di loket
• verb: membantu
• object: saya
• following verb phrase: mengisi formulir

A natural breakdown is:

The lady at the counter | helped | me | fill out the form

So the sentence follows a fairly straightforward Indonesian word order.

Yes, that is possible. Menolong also means to help.

But membantu is usually the most neutral and common choice for this kind of situation.
Menolong can sometimes sound a little more like to assist or to rescue/help someone in need, depending on context.

For routine service situations, membantu feels very natural.

Sometimes yes, depending on the situation, but not literally.

Mbak di loket literally means the lady at the counter. If in context that person is functioning as a receptionist, then The receptionist helped me fill out the form may be a good natural translation.

But if you are being precise about the Indonesian words, the lady at the counter is closer.

Yes, this sounds natural. It is a normal kind of sentence for describing help in an office, clinic, bank, station, government office, or similar place.

It has a natural everyday feel:

• Mbak di loket membantu saya mengisi formulir.

That sounds like something a learner could realistically hear or say.