Learn Hardcore Indonesian: Kami memakai helm saat naik sepeda motor untuk melindungi kepala. - We wear helmets when riding a motorcycle to protect our heads.

kami = we, excluding the person spoken to (listener is not part of the group)
kita = we, including the person spoken to (listener is part of the group)

In the sentence:

Kami memakai helm saat naik sepeda motor untuk melindungi kepala.

the speaker is saying that their group wears helmets, but they are not necessarily including the listener in that group.

If you were talking to someone and wanted to include them, you would say:

Kita memakai helm saat naik sepeda motor untuk melindungi kepala.
= We (you and I / all of us here) wear helmets when riding a motorcycle to protect our heads.

So:

Use kami when the listener is outside the group.
Use kita when the listener is inside the group.

What is the difference between memakai and pakai? Could I say Kami pakai helm?

Both memakai and pakai are commonly used and mean to wear / to use.

memakai = more formal / neutral
pakai = slightly more informal / conversational

In your sentence:

Kami memakai helm... is grammatically perfect and neutral.
Kami pakai helm... is also correct and sounds more casual / everyday.

Other examples:

Formal: Silakan memakai masker. – Please wear a mask.
Informal: Pakai masker, ya. – Wear a mask, okay.

So yes, you can say:

Kami pakai helm saat naik sepeda motor untuk melindungi kepala.

It’s natural in everyday speech.

Why is it kepala and not kepala kami or kepala kami semua? Isn’t the meaning our heads?

Indonesian often omits possessive words (like my, your, our) with body parts and things that obviously belong to the subject.

In English you must say:

We wear helmets to protect our heads.

But in Indonesian, if the subject is clear (kami), you can simply say:

Kami memakai helm ... untuk melindungi kepala.
Literally: We wear helmets ... to protect [the] head.

The listener automatically understands it as our heads.

You can add the possessive if you want to be more explicit:

... untuk melindungi kepala kami. – to protect our heads.

Both are correct. The version without kami is just more typical and natural in everyday Indonesian when talking about body parts belonging to the subject.

Why is kepala singular in Indonesian, while in English we say heads (plural)?

Indonesian nouns usually don’t show plural by changing form. Plural meaning is understood from context.

kepala can mean head or heads, depending on context.

In:

Kami memakai helm ... untuk melindungi kepala.

we know kami refers to more than one person, so kepala is understood as heads.

If you really want to emphasize plurality, you might see:

kepala-kepala kami – our heads (with repetition to emphasize plural)

But in this sentence it would sound unnatural and unnecessarily heavy. The simple kepala is the most natural form.

What does saat mean exactly? Can I replace saat with ketika or waktu here?

saat means when / at the time (that) and introduces a time clause.

In your sentence:

... saat naik sepeda motor ...
... when (we are) riding a motorcycle ...

You can usually replace saat with ketika or waktu without changing the basic meaning:

Kami memakai helm ketika naik sepeda motor...
Kami memakai helm waktu naik sepeda motor...

Nuances:

saat – slightly more formal/neutral (common in writing and speech)
ketika – also neutral, maybe a bit more literary, but used in speech too
waktu – very common in spoken Indonesian, sounds more casual

All three are acceptable in this sentence.

Why is there no subject after saat? Shouldn’t it be saat kami naik sepeda motor?

The subject of the time clause is understood to be the same as the main subject (kami), so Indonesian often omits it to avoid repetition.

Full form (still correct):

Kami memakai helm saat kami naik sepeda motor...

Natural shortened form:

Kami memakai helm saat naik sepeda motor...

Because kami was just stated, saat naik sepeda motor is understood as:

when we are riding a motorcycle

If the subject were different, you would normally need to include it:

Kami memakai helm saat mereka naik sepeda motor.
We wear helmets when they ride motorcycles.

Why is naik used here? I thought naik means to go up or to climb.

You’re right that naik literally means to go up / to ascend, but in Indonesian it is also widely used with vehicles to mean:

to ride / to go by [vehicle]

Common patterns:

naik sepeda – ride a bicycle
naik motor / naik sepeda motor – ride a motorcycle
naik mobil – go by car
naik bus – go by bus
naik pesawat – fly by plane

In this sentence:

saat naik sepeda motor

is understood as:

when (we are) riding a motorcycle / on a motorcycle

So naik + vehicle is a very common way to say "to go by / to ride" that vehicle.

Could I use mengendarai instead of naik? What’s the difference between naik sepeda motor and mengendarai sepeda motor?

Yes, you can use mengendarai, but there is a nuance:

naik sepeda motor
= to ride / be on a motorbike (can be driver or passenger; general)
mengendarai sepeda motor
= to operate/drive a motorbike (focus on controlling the vehicle)

In your sentence:

Kami memakai helm saat naik sepeda motor... – We wear helmets when riding a motorbike.
Kami memakai helm saat mengendarai sepeda motor... – We wear helmets when driving a motorbike.

Both are correct. If you want to emphasize that you are driving (not just riding as a passenger), mengendarai is more precise. But naik is more common and more general in everyday speech.

Is sepeda motor one word or two? And can I just say motor?

sepeda motor is written as two words:

sepeda = bicycle
motor = engine
sepeda motor = "motorized bicycle" → motorcycle / motorbike

In everyday conversation, Indonesians very often shorten it to motor:

naik motor – ride a motorbike
Saya punya motor. – I have a motorbike.

So for your sentence, all of these are possible:

... saat naik sepeda motor ... (neutral, clear)
... saat naik motor ... (very common in speech)

What does untuk do in this sentence? Could I replace it with agar or supaya?

untuk is a preposition that often means for / in order to / to (do something) when followed by a verb.

In:

... untuk melindungi kepala.

it introduces a purpose:

... to protect (our) heads.

So the structure is:

untuk + verb
untuk melindungi – to protect

You can sometimes use agar or supaya to express purpose, but they introduce a clause (need a subject and verb), not just a bare verb:

Kami memakai helm agar kepala kami terlindungi.
Kami memakai helm supaya kepala kami terlindungi.

Here, terlindungi is a passive/adjectival form: "so that our heads are protected."

So:

untuk + V – "to do something" (non-finite verb)
agar/supaya + clause – "so that ..."

In the original sentence, untuk melindungi is the most straightforward choice.

Is this sentence in the present tense? How does Indonesian show tense here?

Indonesian does not change verb forms for tense (past, present, future). The verb memakai here is tenseless; the time meaning comes from context or extra words.

Your sentence can mean:

We wear helmets when riding a motorcycle to protect our heads. (habitual/present)
We were wearing helmets when riding a motorcycle to protect our heads. (past, if context is past)
We will wear helmets when riding a motorcycle to protect our heads. (future, with context)

To add explicit time, Indonesians use time expressions:

Kemarin kami memakai helm saat naik sepeda motor... – Yesterday we wore helmets...
Besok kami akan memakai helm saat naik sepeda motor... – Tomorrow we will wear helmets...

The verb memakai itself doesn’t change.

Can I change the word order to start with saat? Is this still correct?

Yes, Indonesian word order is flexible for clauses like this. You can front the time clause:

Original:

Kami memakai helm saat naik sepeda motor untuk melindungi kepala.

Alternative:

Saat naik sepeda motor, kami memakai helm untuk melindungi kepala.

Both are correct and natural. Starting with Saat... slightly emphasizes the condition/time:

When riding a motorcycle, we wear helmets to protect our heads.

Is helm the normal word for helmet? Is there any difference from topi?

Yes, helm is the standard word for helmet (motorbike, construction, safety helmets, etc.).

helm – helmet (protective headgear)
topi – cap/hat (non‑protective, for fashion, sun, etc.)

So:

memakai helm – wear a helmet
memakai topi – wear a hat/cap

In your sentence, helm is exactly the right word.

Could I add something to show that we must wear helmets, not just that we do?

Yes, you can add harus (must / have to) before the verb:

Kami harus memakai helm saat naik sepeda motor untuk melindungi kepala.
= We must wear helmets when riding a motorcycle to protect our heads.

Pattern:

harus + verb – must / have to do something

You can also use wajib (obligatory, often for legal or very strong obligation):

Kami wajib memakai helm saat naik sepeda motor.
= We are obligated to wear helmets when riding a motorcycle.