Learn Hardcore Indonesian: Polisi berpatroli di sekitar sekolah untuk menjaga keamanan siswa. - The police patrol around the school to maintain students’ safety.

The base word is patroli (patrol).
The prefix ber- often turns a noun into an intransitive verb meaning to do that activity or to have that thing.

So:

patroli = patrol (noun)
berpatroli = to patrol / to be on patrol

In this sentence, Polisi berpatroli corresponds to English The police patrol or The police are patrolling.

Is polisi singular or plural here? How do you say a police officer versus the police?

Indonesian usually does not mark singular vs plural on the noun itself, so polisi can mean:

a police officer
police officers
the police (as an institution)

Context tells you which is meant. In this sentence, it most naturally means the police (as a group or institution).

To be more specific:

seorang polisi = a police officer (literally: one person of police)
beberapa polisi = several police officers
para polisi = the police officers (emphasizing the plurality)
polisi itu = that police officer / those police officers, depending on context

There is no the or a in the sentence. How does Indonesian show definiteness?

Indonesian does not use articles like a, an, or the.

Definiteness is expressed mainly by:

Context
- polisi = a police officer / the police — depends on what makes sense in context.
Determiners when needed
- ini = this
- itu = that
  Example: polisi itu (that police officer / that police)
Quantifiers
- seorang polisi = a police officer
- tiga polisi = three police officers

In this sentence, Polisi berpatroli... is naturally understood as The police patrol..., even though there is no explicit the.

What does di sekitar sekolah mean exactly, and why use di and sekitar together?

Di is a preposition meaning at / in / on, and sekitar means around / surrounding.

So:

di sekolah = at the school
sekitar sekolah = the area around the school
di sekitar sekolah = in the area around the school / around the school

Using di sekitar together emphasizes a location in the surroundings of something.
Sekitar sekolah alone tends to be more like a noun phrase (the area around the school), while di sekitar sekolah is a full prepositional phrase (around the school).

Could the sentence use only sekitar sekolah without di? Would it still be correct?

You could say Polisi berpatroli sekitar sekolah, and many Indonesians would still understand you. However:

The most natural and standard form is di sekitar sekolah.
Without di, it sounds a bit more informal or slightly elliptical.

For learners, it is safer and more idiomatic to include di:
berpatroli di sekitar sekolah.

What does untuk do in untuk menjaga keamanan siswa? Can it be omitted?

Untuk means for or to when expressing purpose.

menjaga = to guard, to maintain, to protect
keamanan siswa = the security/safety of the students

So untuk menjaga keamanan siswa = to maintain the security of the students / in order to protect the students’ safety.

About omitting untuk:

Polisi berpatroli menjaga keamanan siswa is possible in some contexts, but it sounds less clear and less standard.
With verbs of movement or activity, untuk is usually included to clearly mark purpose: berpatroli untuk menjaga...

For clear, natural Indonesian, keep untuk here.

What is the difference between keamanan and keselamatan?

Both can be translated as safety, but they are used differently:

aman = safe
- keamanan = security, safety in the sense of protection from crime, threats, disturbance.
  - Example meanings: security guards, crime prevention, public order.
selamat = safe, saved, unharmed
- keselamatan = safety in the sense of not getting injured or killed, especially in accidents, disasters, etc.

In this sentence:

keamanan siswa focuses on security (e.g., from crime, violence, threats) around the school.
keselamatan siswa would emphasize physical safety from accidents, disasters, and so on.

Both are understandable, but keamanan siswa fits well with police patrolling.

What exactly does menjaga mean here, and why not simply say melindungi?

Menjaga literally means to guard, to watch over, to look after, or to maintain.

menjaga keamanan siswa = to guard/maintain the security of the students.

Melindungi means to protect (from harm).
You could say melindungi siswa (to protect the students), but:

menjaga keamanan siswa focuses on maintaining a secure, orderly situation.
melindungi siswa focuses more directly on protecting the students from specific dangers.

With police patrolling, menjaga keamanan is a very common collocation, so it sounds more natural.

Does siswa mean student? How is it different from murid or pelajar?

Yes, siswa means student, but there are some nuances:

siswa
- Commonly used for school students, especially at elementary, junior high, and senior high school.
- Often used in formal or official contexts: keamanan siswa, hak-hak siswa.
murid
- Also means student or pupil.
- Can sound slightly more traditional or informal, and sometimes suggests a closer teacher–pupil relationship.
pelajar
- Student, but more often used for those in junior high, high school, or sometimes any young learner; can also refer to students in a more general social category (for example, demonstrasi pelajar = student demonstrations).

In this sentence, siswa is very natural for school students.

What does the part keamanan siswa literally mean?

Breakdown:

keamanan
- From aman (safe) + ke- -an noun-forming affix.
- Means safety, security, state of being safe.
siswa = students.

Keamanan siswa literally means the security/safety of the students.
So the whole phrase menjaga keamanan siswa is to guard/maintain the students’ security.

Could we say Polisi melakukan patroli di sekitar sekolah instead of Polisi berpatroli di sekitar sekolah? Is there a nuance difference?

Yes, you can say Polisi melakukan patroli di sekitar sekolah, and it is grammatically correct and natural.

Nuance:

berpatroli
- Single verb, concise, very common in news reports and formal writing.
- Feels slightly more compact and neutral.
melakukan patroli = to do conduct a patrol
- Slightly more formal or heavier in style because it is a verb phrase formed with melakukan (to do, to carry out) + noun patroli.

Both mean the police patrol around the school. In everyday use, berpatroli is simpler and very typical.

Why is the verb not marked for tense? How do we know if it means patrolled, are patrolling, or will patrol?

Indonesian verbs are not inflected for tense. Berpatroli itself does not show past, present, or future.

Time reference is understood from context or from time words, for example:

kemarin polisi berpatroli = yesterday the police patrolled
sekarang polisi berpatroli = now the police are patrolling
besok polisi akan berpatroli = tomorrow the police will patrol

In the given sentence, without extra time information, it can generally be understood as a present or habitual action:

The police patrol around the school (regularly / as a routine).
The police are patrolling around the school (currently), depending on context.