Guru menjelaskan dasar matematika di kelas.

Indonesian does not normally use articles like the or a/an.

So guru can mean:

• a teacher
• the teacher
• sometimes just teacher/teachers depending on context

The exact meaning usually comes from the situation, not from an article.

If you really want to make it more specific, Indonesian can add words like:

• seorang guru = a teacher
• guru itu = the teacher / that teacher

In your sentence, Guru menjelaskan dasar matematika di kelas, guru is perfectly natural without any article.

By itself, guru is neutral. It does not automatically tell you whether it is:

• singular or plural
• definite or indefinite

So depending on context, it could be understood as:

• The teacher explains...
• A teacher explains...
• less commonly, Teachers explain...

In a simple learning sentence like this, English often translates it as The teacher explains the basics of mathematics in class or A teacher explains basic mathematics in class.

Menjelaskan comes from the root jelas, which means clear.

When Indonesian adds meN- and -kan, it becomes a verb meaning:

• to explain
• literally something like to make clear

So:

• jelas = clear
• menjelaskan = to explain / to make something clear

This is a very common pattern in Indonesian:

• root word
• plus affixes
• becomes a more specific verb

Because jelas is not the right form for the action here.

• jelas is usually an adjective: clear
• menjelaskan is the verb: to explain

Compare:

• Penjelasannya jelas. = The explanation is clear.
• Guru menjelaskan materi. = The teacher explains the material.

In your sentence, the teacher is doing an action, so the verb menjelaskan is needed.

It can be understood as the basics/fundamentals of mathematics or basic mathematics, depending on context.

Word by word:

• dasar = base, basis, foundation, fundamentals
• matematika = mathematics / math

Together, dasar matematika usually means:

• basic mathematics
• the basics of mathematics
• mathematical fundamentals

A very natural English translation here is the basics of mathematics or basic math.

Indonesian often links nouns directly without a separate word like of.

So:

• dasar matematika literally looks like foundation mathematics
• but it means foundation of mathematics or mathematical basics

This noun + noun pattern is very common in Indonesian:

• buku sejarah = history book
• guru matematika = math teacher
• dasar matematika = basics of mathematics / mathematical basics

English often needs of or an adjective, but Indonesian often does not.

It can mean either, depending on context.

• di = in / at
• kelas = class / classroom

So di kelas may mean:

• in class
• in the classroom
• during class, in some contexts

In your sentence, both in class and in the classroom are possible translations.

Because this di is a preposition, meaning in, at, or on.

Prepositional di is written separately:

• di kelas = in class
• di rumah = at home
• di sekolah = at school

But there is also di- as a prefix for passive verbs, and that is written together:

• ditulis = written
• dibaca = read
• dijelaskan = explained

So:

• di kelas = correct, separate, because it is a preposition
• dikelas = incorrect in this sentence

Mostly yes. The basic order is:

• Guru = subject
• menjelaskan = verb
• dasar matematika = object
• di kelas = place

So the structure is:

Subject + Verb + Object + Place

That is very similar to English: The teacher explains basic mathematics in class.

This makes many simple Indonesian sentences feel familiar to English speakers.

Not specifically. Indonesian verbs do not change the way English verbs do for tense.

So menjelaskan by itself does not mean only:

• explains
• is explaining
• explained
• will explain

The time usually comes from context or from time words such as:

• sekarang = now
• kemarin = yesterday
• besok = tomorrow
• sedang = currently / in the process of
• akan = will

Without extra context, this sentence is often translated in the simple present: The teacher explains...

But depending on context, it could also describe a general habit or a current action.

Yes, in the right context it could.

Indonesian often leaves aspect unstated unless it needs to be clear. If you wanted to make the ongoing action more explicit, you could say:

• Guru sedang menjelaskan dasar matematika di kelas.

That means:

• The teacher is explaining the basics of mathematics in class.

Without sedang, the original sentence is more neutral.

By default, kelas is just the basic noun class/classroom. Indonesian nouns do not automatically show singular or plural the way English does.

So context decides whether it means:

• class
• the class
• the classroom
• in some situations even classes

If Indonesian really wants to show plural clearly, it can use reduplication:

• kelas-kelas = classes

But in your sentence, di kelas is most naturally understood as in class or in the classroom.

Because Indonesian often expresses these relationships simply by placing nouns together.

So dasar matematika does the job without an extra possessive marker.

English needs something like:

• the basics of mathematics
• mathematical basics
• basic mathematics

Indonesian can simply say:

• dasar matematika

This is one of the most common things English speakers need to get used to in Indonesian.

Yes, but it is slightly different in feel.

• dasar matematika = the basics/foundations of mathematics
• matematika dasar = basic mathematics / elementary math

Both are possible, but they are not always identical in nuance.

Very roughly:

• dasar matematika focuses more on the foundations/basics
• matematika dasar sounds more like a basic-level math subject

So your original sentence is good if the idea is the teacher explains the fundamentals of math.

Yes, it is natural and grammatical.

It is a straightforward sentence with:

• a subject: guru
• an active verb: menjelaskan
• an object: dasar matematika
• a location: di kelas

A slightly more specific version could be:

• Guru itu menjelaskan dasar matematika di kelas. = The teacher explains the basics of mathematics in class.
• Seorang guru menjelaskan dasar matematika di kelas. = A teacher explains the basics of mathematics in class.

But the original sentence is perfectly normal Indonesian.