Saya berjanji tidak akan terlambat lagi.

Yes. Saya means I and is the more neutral or polite choice.

You can often replace it with aku in casual speech:

• Saya berjanji tidak akan terlambat lagi.
• Aku berjanji tidak akan terlambat lagi.

Both mean the same thing, but aku sounds more informal and personal.

Janji is the base word meaning promise.

With the prefix ber-, berjanji becomes a verb meaning to make a promise / to promise.

So:

• janji = promise
• berjanji = to promise

In everyday Indonesian, people also very often say saya janji in casual speech, even though berjanji is the more standard full verb.

The prefix ber- often forms an intransitive verb, often meaning something like to do, to have, or to engage in whatever the base word suggests.

In berjanji, it turns the noun-like base janji into the verb to promise.

Other examples:

• bicara → berbicara = to speak
• main → bermain = to play
• kerja → bekerja = to work

So here, ber- is part of making the natural verb form.

Because in Indonesian, tidak usually comes before the word or phrase it negates.

Here, akan terlambat means will be late, and tidak akan terlambat means will not be late.

So the structure is:

• akan terlambat = will be late
• tidak akan terlambat = will not be late

Putting akan before tidak would sound unnatural in standard Indonesian.

Akan marks future meaning, similar to will in English.

So:

• terlambat = late
• akan terlambat = will be late
• tidak akan terlambat = will not be late

Indonesian often leaves future meaning unstated if the context is already clear, so you may also hear:

• Saya berjanji tidak terlambat lagi.

But akan makes the future promise clearer and more explicit.

Because tidak negates verbs and adjectives, while bukan usually negates nouns or noun phrases.

In this sentence:

• berjanji is a verb
• terlambat behaves like an adjective/state

So tidak is correct:

• tidak akan terlambat

Compare:

• Saya tidak sibuk. = I am not busy.
• Dia bukan guru. = He/She is not a teacher.

Terlambat means late.

Yes, the ter- is part of the word. In this case, learners should treat terlambat as the normal word for late rather than trying to break it apart too much.

Examples:

• Saya terlambat. = I am late.
• Dia datang terlambat. = He/She came late.

Although ter- can have several functions in Indonesian, in terlambat it is best learned as a fixed vocabulary item.

Here, lagi means again / anymore, depending on how you translate it naturally into English.

In this sentence:

• tidak akan terlambat lagi = won’t be late again / won’t be late anymore

This use of lagi often shows that something happened before and the speaker says it will not happen from now on.

So it suggests: I have been late before, but I promise it won’t happen again.

The order is very natural for Indonesian:

• Saya = subject
• berjanji = main verb
• tidak akan terlambat lagi = what is being promised

So literally, the structure is something like:

• I promise [not will late again]

A more natural English rendering is:

• I promise I won’t be late again.

Indonesian often does not need an extra word like that or a second subject such as I inside the second clause when the meaning is already clear.

Yes, you could, and it would be understandable.

Adding the second saya makes the embedded clause more explicit:

• Saya berjanji saya tidak akan terlambat lagi.

But in most situations, it is more natural and less repetitive to omit it:

• Saya berjanji tidak akan terlambat lagi.

Indonesian often leaves out repeated subjects when they are obvious from context.

It is mostly neutral to slightly formal, especially because of saya and berjanji.

Different levels:

• Saya berjanji tidak akan terlambat lagi. = neutral/standard
• Aku berjanji tidak akan terlambat lagi. = informal
• Aku janji nggak bakal telat lagi. = very casual spoken Indonesian

Notes on the casual version:

• nggak = informal not
• bakal = informal will / going to
• telat = casual form of terlambat

Yes, often it can.

You may hear:

• Saya berjanji tidak terlambat lagi.

This can still be understood as I promise not to be late again because the idea of a promise already points toward the future.

However, akan is useful when you want to make the future meaning very clear:

• Saya berjanji tidak akan terlambat lagi.

So both are possible, but the version with akan is especially clear and standard.

A very natural casual spoken version would be:

• Aku janji nggak bakal telat lagi.

This is common in conversation. Compared with the original:

• saya → aku
• berjanji → janji
• tidak → nggak
• akan → bakal
• terlambat → telat

The original sentence is still perfectly correct and useful, especially in writing or polite speech.