Tolong masukkan surat itu ke amplop cokelat sebelum rapat dimulai.

Tolong makes the sentence more polite. In this kind of sentence, it works a lot like please in English.

• Tolong masukkan... = Please put...
• Masukkan... without tolong is still grammatical, but it sounds more direct, like a plain command.

Literally, tolong can also mean help, but in requests like this, its main job is to soften the imperative.

Because masuk and masukkan do different jobs.

• masuk = to enter / go in
  This is usually intransitive, so the thing itself goes in.
• memasukkan = to put/insert something into something
  This is transitive, so it takes an object.

In a command, memasukkan becomes masukkan.

So:

• Surat itu masuk ke amplop = The letter goes into the envelope
• Masukkan surat itu ke amplop = Put the letter into the envelope

Here, someone is being told to place the letter in the envelope, so masukkan is the correct form.

Yes. Indonesian commands often leave out the subject, just like English can.

The understood subject is you:

• (Anda/Kamu), tolong masukkan surat itu...

Usually you do not need to say Anda or kamu unless you want extra emphasis, contrast, or clarity. Leaving it out sounds normal and natural.

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

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

So surat itu is the normal word order.

Itu surat can exist, but it usually has a different function, such as pointing something out or adding emphasis, like That is a letter or That letter... in a marked context. It is not the normal neutral way to say that letter.

It can feel like either one, depending on context.

In many sentences, itu literally means that, but it can also make a noun feel definite, like the in English. So surat itu may be understood as:

• that letter
• the letter already known in the situation

Indonesian does not have articles exactly like English a/an/the, so words like ini and itu often help show whether something is specific.

Because adjectives usually come after the noun in Indonesian.

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

So the order is:

• noun + adjective

Also, cokelat can mean brown or chocolate, depending on context. Here it clearly means the color brown.

Because ke shows movement toward a destination, while di shows location.

• ke = to / into / toward
• di = in / at / on

Here, the letter is being moved into the envelope, so ke is the right choice.

• masukkan surat itu ke amplop = put the letter into the envelope
• surat itu di amplop would sound like the letter is located in/on the envelope, which is not what you want here

Also, many speakers would say ke dalam amplop cokelat for extra clarity. That is also very natural.

Dimulai is the passive form of mulai.

• mulai = begin / start
• memulai = to start something
• dimulai = to be started / is started

So:

• rapat dimulai = the meeting is started / the meeting starts

In English, we often just say before the meeting starts. In Indonesian, sebelum rapat dimulai is a very natural way to express that idea.

Yes, you could say sebelum rapat mulai, and many people do in everyday speech.

The difference is mostly style:

• sebelum rapat dimulai = more standard, complete, and a bit more formal
• sebelum rapat mulai = more conversational and less formal

In office, written, or careful spoken Indonesian, dimulai sounds very normal. In casual conversation, mulai is also common.

Yes, this sentence sounds polite and fairly standard. It fits well in a workplace or other semi-formal setting.

A more casual spoken version might be:

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

Here:

• masukin is a colloquial spoken form of masukkan
• mulai is less formal than dimulai

So the original sentence is a good model for standard Indonesian, while the casual version is closer to everyday speech.

Yes. Indonesian word order is flexible enough for that.

You can say:

• Sebelum rapat dimulai, tolong masukkan surat itu ke amplop cokelat.

This has the same basic meaning. Putting the time phrase first gives a little more emphasis to the deadline or timing.

Both versions are natural:

• Tolong masukkan surat itu ke amplop cokelat sebelum rapat dimulai.
• Sebelum rapat dimulai, tolong masukkan surat itu ke amplop cokelat.