Saya lupa menyalakan sein ketika belok kanan.

Yes, Indonesian often drops the subject when it is clear from context. So Lupa menyalakan sein ketika belok kanan can sound natural in conversation if everyone already knows who is being talked about.

But Saya lupa menyalakan sein ketika belok kanan is a complete, clear sentence and is especially natural in neutral or careful speech. Including saya makes it explicit: I forgot to turn on the turn signal when turning right.

Lupa means to forget or forgot.

A very common pattern is:

lupa + verb

So:

• lupa menyalakan sein = forgot to turn on the turn signal
• lupa membawa dompet = forgot to bring a wallet
• lupa makan = forgot to eat

This is simpler than English in some ways, because Indonesian does not need a separate word like to before the second verb.

Menyalakan means to switch on / turn on something.

It comes from nyala related to being on / lit, and with the prefix-suffix combination meN- ... -kan, it becomes a transitive verb: to turn something on.

Compare:

• lampunya menyala = the lamp is on / is lit
• saya menyalakan lampu = I turned on the lamp

So in your sentence:

• menyalakan sein = to turn on the turn signal

In casual speech, people may say nyalain instead of menyalakan:

• Saya lupa nyalain sein = I forgot to turn on the turn signal

That version is more informal.

Sein means turn signal, indicator, or blinker in everyday Indonesian.

Very often people say simply:

• sein kiri = left turn signal
• sein kanan = right turn signal

You may also hear:

• lampu sein = turn signal light

So menyalakan sein and menyalakan lampu sein are both understandable. The version without lampu is very common in speech.

Indonesian often leaves out repeated subjects when they are already obvious.

So:

• ketika belok kanan = when turning right
• ketika saya belok kanan = when I was turning right

Both are possible. The shorter version sounds natural because the subject is already understood to be saya from the earlier part of the sentence.

This kind of omission is very common in Indonesian.

In everyday Indonesian, belok kanan is the normal way to say turn right.

Examples:

• belok kiri = turn left
• belok kanan = turn right

Although forms like membelok exist, they are less common in this kind of everyday driving instruction. Belok kanan is the natural fixed expression.

So ketika belok kanan sounds much more normal than ketika membelok ke kanan in ordinary speech.

Yes. Both are possible, but they are used a little differently.

Common options:

• belok kanan = turn right
• belok ke kanan = turn to the right

Both are grammatical. In driving contexts, belok kanan is very common and compact.

So you could also say:

• Saya lupa menyalakan sein ketika belok ke kanan.

That sounds fine too.

Indonesian does not mark tense the way English does. Verbs do not change form for past, present, or future.

So lupa can mean:

• forget
• forgot

The time is understood from context. In this sentence, the most natural reading is past because it describes a completed event: I forgot to turn on the signal when turning right.

If needed, Indonesian can add time words such as:

• tadi = earlier / just now
• kemarin = yesterday
• barusan = just now

Example:

• Tadi saya lupa menyalakan sein ketika belok kanan.

Yes.

All of these can mean when in this sentence:

• ketika
• saat
• waktu

Examples:

• Saya lupa menyalakan sein ketika belok kanan.
• Saya lupa menyalakan sein saat belok kanan.
• Saya lupa menyalakan sein waktu belok kanan.

General tendency:

• ketika sounds a bit more neutral or written
• saat is also common and natural
• waktu is very common in speech

All three work here.

It is mostly neutral.

Why:

• saya is neutral to polite
• menyalakan is standard
• ketika is standard
• sein is normal everyday vocabulary

A more casual spoken version might be:

• Aku lupa nyalain sein waktu belok kanan.

Differences:

• aku instead of saya = more informal
• nyalain instead of menyalakan = more colloquial
• waktu instead of ketika = more conversational

This is a very natural order:

Saya lupa menyalakan sein ketika belok kanan.

It follows a common pattern:

subject + lupa + verb phrase + time clause

You can move parts around for emphasis, but the original order is the most straightforward.

For example:

• Ketika belok kanan, saya lupa menyalakan sein.

This is also correct and means the same thing, but it puts emphasis first on the situation when turning right.

Yes, but the meaning changes a little.

Your sentence:

• Saya lupa menyalakan sein ketika belok kanan.
• Meaning: I forgot to turn on the signal when turning right.

Another sentence:

• Saya tidak menyalakan sein ketika belok kanan.
• Meaning: I did not turn on the signal when turning right.

The second one only says the action did not happen. It does not say why. The first one specifically says it was because of forgetting.

So lupa adds the idea of unintentional failure.

Yes, sein is a borrowed word, and that is completely normal. Indonesian uses many loanwords, especially for everyday objects, technology, and transport.

For a learner, the important thing is that sein is a very common real-life word in Indonesian, especially in traffic and driving contexts. So it is absolutely worth learning as standard everyday vocabulary.