Learn Hardcore Malay: Guru sains mengajar pelajar di sekolah setiap pagi. - The science teacher teaches students at school every morning.

Also, guru sains by itself is not clearly a or the; it can be either, depending on context (Malay has no articles like a / the by default).

How would I say a science teacher versus the science teacher in Malay?

Malay often leaves this vague, but if you want to be clear:

a science teacher
→ seorang guru sains
(seorang = a / one (person), used for people)
the science teacher (specific, known from context)
→ guru sains itu
(itu = that / the, marking something specific already known)

Examples:

Seorang guru sains mengajar pelajar di sekolah setiap pagi.
= A science teacher teaches students at school every morning.
Guru sains itu mengajar pelajar di sekolah setiap pagi.
= The science teacher (that we know about) teaches students at school every morning.

Why is it mengajar and not just ajar?

The base word is ajar (to teach). Malay often adds the prefix meN- to make an active verb:

ajar = teach (root; also used in imperatives), e.g.
Ajar saya! = Teach me!
mengajar = to teach (normal verb form in sentences)

In this sentence, you need the meN- form because it is a normal verb in a statement:

Guru sains mengajar pelajar... ✅ (natural)
Guru sains ajar pelajar... ✅ (colloquial / informal speech)
In informal spoken Malay you will hear ajar used like a regular verb, but mengajar is the standard/politer form in writing and formal speech.

Do I need a word like kepada (to) after mengajar, as in teaches to the students?

No, you do not need kepada here.

mengajar pelajar already means teach students.
The object (pelajar) comes directly after the verb.

So:

Guru sains mengajar pelajar... ✅
Guru sains mengajar kepada pelajar... ⚠️ sounds awkward here

You might see mengajar sesuatu kepada seseorang in more complex sentences:

Dia mengajar matematik kepada pelajar-pelajarnya.
= He/she teaches mathematics to his/her students.

But for a simple sentence like yours, mengajar pelajar is correct and natural.

What is the difference between pelajar and murid?

Both can translate as student, but there is a typical usage difference:

murid
- More common for primary-school pupils (young children).
- Still understood more generally as “student”, but has a “schoolkid” feel.
pelajar
- More common for secondary school, college, university students.
- Feels a bit more grown-up / general.

In your sentence, pelajar could mean “students” in a general sense, or specifically older students depending on context.

Plural is usually not marked; pelajar can mean student or students. If you want to emphasize plural:

pelajar-pelajar (reduplication)
para pelajar (formal: the students / all the students)

Why is it di sekolah and not ke sekolah or pada sekolah?

These prepositions have different functions:

di = at / in / on (location, no movement)
- di sekolah = at school / in school
ke = to (movement towards a place)
- ke sekolah = to school (going to school)
pada = at / on / to, but mainly for time or abstract objects, not for physical location like a building
- pada pukul 8 = at 8 o’clock
- pada pendapat saya = in my opinion

So in your sentence, you want a static location (teaches at school), so di sekolah is correct.

Can I move setiap pagi to another place in the sentence?

Yes. Malay is quite flexible with time expressions. All of these are grammatical:

Guru sains mengajar pelajar di sekolah setiap pagi.
(Neutral, very natural.)
Setiap pagi, guru sains mengajar pelajar di sekolah.
(Emphasizes every morning.)
Guru sains setiap pagi mengajar pelajar di sekolah.
(Possible, but less common; sounds a bit marked in writing.)

Most common patterns:

Time at the end:
... di sekolah setiap pagi.
Time at the beginning:
Setiap pagi, ...

Both are fine; choose based on what you want to emphasize.

How is tense shown here? How do I know this is present tense and not past or future?

Malay verbs usually do not change form for tense. Mengajar can mean:

teaches / is teaching
taught
will teach

The tense is understood from context and time expressions like setiap pagi (every morning), semalam (yesterday), esok (tomorrow), etc.

In this sentence:

setiap pagi (every morning) suggests a habitual present:
The science teacher teaches students at school every morning.

If you really need to mark tense, you can add particles:

telah / sudah mengajar = has taught / already taught
sedang mengajar = is teaching (right now)
akan mengajar = will teach

But here, mengajar alone + setiap pagi is enough.

Are guru sains and pelajar singular or plural in this sentence?

By default, Malay nouns are number-neutral: they can be singular or plural.

guru sains = a science teacher / the science teacher / science teachers (depending on context)
pelajar = student / students

In your sentence, the most natural reading is:

guru sains = one teacher (because the verb is just singular in English),
pelajar = multiple students (by real-world logic).

If you want to force the meanings:

Seorang guru sains mengajar para pelajar di sekolah setiap pagi.
= A science teacher teaches the students at school every morning.
Para guru sains mengajar para pelajar di sekolah setiap pagi.
= The science teachers teach the students at school every morning.

Does di sekolah describe where he teaches, or which students (students who are at that school)?

In Guru sains mengajar pelajar di sekolah setiap pagi, di sekolah is most naturally understood as the place of the action:

The science teacher teaches students *at school every morning.*

It is describing mengajar (teaches), not tightly specifying which students.

If you want to clearly say students of that school, Malay often uses yang or a slightly different structure, for example:

Guru sains mengajar pelajar-pelajar sekolah itu setiap pagi.
= The science teacher teaches the students of that school every morning.
Guru sains mengajar pelajar yang belajar di sekolah itu setiap pagi.
= The science teacher teaches the students who study at that school every morning.

In everyday speech, pelajar di sekolah can be a bit ambiguous, but context usually makes it clear.

Would this sentence sound different in casual spoken Malay? How might people actually say it?

The original sentence is perfectly natural and a bit neutral/formal. In everyday speech, you might hear:

Cikgu sains ajar murid-murid di sekolah setiap pagi.
- cikgu is the common spoken word for teacher (especially schoolteachers).
- ajar instead of mengajar (informal).
- murid-murid emphasizes plural (the pupils).

Or, with time at the start:

Setiap pagi, cikgu sains ajar murid di sekolah.

So:

Guru sains mengajar pelajar di sekolah setiap pagi.
→ neutral / written / formal-ish.
Cikgu sains ajar murid di sekolah setiap pagi.
→ casual spoken version with the same basic meaning.