Saya memasukkan surat itu ke amplop cokelat sebelum rapat dimulai.

Masuk means to enter / to go in and is usually intransitive, so it does not normally take a direct object.

• Saya masuk. = I go in / enter.

But in this sentence, the speaker is causing the letter to go into the envelope. For that, Indonesian uses memasukkan, which means to put in / insert.

• Saya memasukkan surat itu ke amplop. = I put that letter into the envelope.

So:

• masuk = the subject goes in
• memasukkan = the subject puts something in

It comes from:

• root: masuk = enter, go in
• prefix: meN-
• suffix: -kan

So:

• masuk → memasukkan

This often gives the idea to cause something to enter or to put something into something.

The doubled k appears because the root already ends in k (masuk) and then -kan is added.

Surat itu means that letter or the letter depending on context.

In Indonesian, demonstratives like ini and itu usually come after the noun:

• surat itu = that letter
• amplop ini = this envelope

This is different from English, where this/that comes before the noun.

Also, itu can sometimes feel like that and sometimes more like the, especially when the thing is already known in context.

Ke marks direction or destination: to, into, toward.

Here, the letter is being moved into the envelope, so ke shows where it is being put:

• ke amplop cokelat = into the brown envelope

A slightly more explicit version would be:

• ke dalam amplop cokelat = into the inside of the brown envelope

But in many contexts, ke amplop is perfectly natural because the verb memasukkan already makes the meaning clear.

In Indonesian, adjectives usually come after the noun.

So:

• amplop cokelat = brown envelope
• rumah besar = big house
• buku baru = new book

That is why cokelat follows amplop.

It can mean either, depending on context.

• as a noun: cokelat = chocolate
• as a color adjective: cokelat = brown

In amplop cokelat, it clearly means brown, because it describes the envelope’s color.

Sebelum means before.

It introduces a time clause:

• sebelum rapat dimulai = before the meeting started / before the meeting began

So the sentence structure is:

• Saya memasukkan surat itu ke amplop cokelat = main action
• sebelum rapat dimulai = when that action happened, relative to another event

Dimulai is the passive form of memulai (to begin something).

• rapat dimulai literally means the meeting was begun / the meeting started

This is a very common and natural way to talk about events beginning in Indonesian.

You may also hear rapat mulai, especially in everyday speech, but rapat dimulai sounds a bit more standard and polished in this kind of sentence.

So:

• rapat dimulai = very natural, standard
• rapat mulai = also possible in some contexts, often more conversational

Di- is a common marker of the passive voice in Indonesian.

Compare:

• Mereka memulai rapat. = They begin the meeting.
• Rapat dimulai. = The meeting is begun / The meeting starts.

In the sentence you gave, the focus is on the meeting starting, not on who started it, so the passive form dimulai is used.

A simple breakdown is:

• Saya = subject
• memasukkan = verb
• surat itu = direct object
• ke amplop cokelat = prepositional phrase showing destination
• sebelum rapat dimulai = time clause

So the core pattern is:

Subject + Verb + Object + Destination + Time clause

That is a very normal Indonesian sentence structure.

Yes. Indonesian is flexible here.

You can also say:

• Sebelum rapat dimulai, saya memasukkan surat itu ke amplop cokelat.

This means the same thing. Putting the time clause first gives it a bit more emphasis, similar to English:

• Before the meeting started, I put the letter into the brown envelope.

Both orders are natural.

Yes, this version sounds neutral to fairly standard.

In casual speech, people might say something like:

• Saya masukin surat itu ke amplop cokelat sebelum rapat mulai.

Here:

• masukin is a colloquial form of memasukkan
• rapat mulai sounds more conversational than rapat dimulai

So the original sentence is a good model for standard Indonesian, while the casual version is more everyday spoken style.

Surat often means letter, but in Indonesian it can also refer to various kinds of written documents, depending on context.

For example:

• surat cinta = love letter
• surat resmi = official letter
• surat izin = permission letter
• surat kabar = newspaper

In your sentence, because it is being put into an envelope, letter is the most natural interpretation.