Learn Hardcore Indonesian: Guru menjelaskan teknik dasar di kelas. - The teacher explains basic techniques in class.

Indonesian normally does not use articles like the or a. A bare noun like guru can mean a teacher, the teacher, or teachers depending on context.

If you really need to be more specific, you can add other words:

seorang guru = a teacher (one teacher, not specified)
guru itu = that teacher / the teacher (a specific one already known)
para guru = the teachers (group of teachers)

In everyday sentences, just guru is usually enough, and listeners use context to understand whether it is a or the.

What is the base word of menjelaskan, and what do me- and -kan mean?

The base word (root) is jelas, which means clear.

The verb menjelaskan is formed like this:

jelas (clear)
me-
- jelas
  - -kan → menjelaskan

The combination me- … -kan often makes a transitive, causative verb: to make something become X.

So:

jelas = clear
menjelaskan = to make something clear → to explain (something)

That is why menjelaskan teknik dasar means to explain basic techniques.

Could I just say guru menjelas teknik dasar without -kan?

No. menjelas is not a standard verb in Indonesian. The correct and natural form is menjelaskan.

Some roots can appear with me- alone, some with me- … -kan, some with me- … -i, and some with more than one pattern, but jelas normally becomes menjelaskan when used as a verb meaning to explain.

So you should say:

Guru menjelaskan teknik dasar. ✅
Guru menjelas teknik dasar. ❌ (ungrammatical)

How do I know if this sentence is in the past, present, or future?

Indonesian verbs do not change form for tense. Guru menjelaskan teknik dasar di kelas can mean:

The teacher explains basic techniques in class. (habitual / present)
The teacher explained basic techniques in class. (past)
The teacher will explain basic techniques in class. (future, less common without extra words)

To make the time clearer, Indonesians add time expressions or aspect markers, for example:

Kemarin guru menjelaskan teknik dasar di kelas. = Yesterday the teacher explained …
Sekarang guru menjelaskan teknik dasar di kelas. = Now the teacher is explaining …
Besok guru akan menjelaskan teknik dasar di kelas. = Tomorrow the teacher will explain …
Guru sedang menjelaskan teknik dasar di kelas. = The teacher is explaining (right now).

What is the difference between menjelaskan and mengajar?

Both involve teaching, but the focus is different:

mengajar = to teach (in general; to give instruction)
menjelaskan = to explain (to make something clear)

Examples:

Guru mengajar matematika. = The teacher teaches math.
Guru menjelaskan rumus matematika. = The teacher explains a math formula.

So in your sentence, menjelaskan emphasizes the act of explaining specific content (the basic techniques), not general teaching activity.

What does di in di kelas mean? Is it like in or at?

di is a general preposition for location and can often be translated as in, at, or on, depending on context.

di kelas can be understood as in class, in the classroom, or in the class.
It focuses on where something happens, not movement.

Compare:

di kelas = in/at the class (location)
ke kelas = to the class / to the classroom (movement toward a place)

Is there a difference between di kelas and di dalam kelas?

Yes, but it is subtle:

di kelas is the normal, neutral way to say in class / in the classroom or during class.
di dalam kelas literally means inside the class / inside the classroom, and it slightly emphasizes being inside as opposed to outside.

In many situations, they are interchangeable, but di dalam kelas sounds a bit more explicit or descriptive about the inside location:

Guru menjelaskan teknik dasar di kelas. = The teacher explains basic techniques in class.
Guru menjelaskan teknik dasar di dalam kelas. = The teacher explains basic techniques inside the classroom (as opposed to, say, in the hallway).

Why is dasar after teknik? In English we say basic technique, not technique basic.

In Indonesian, descriptive words (adjectives) usually come after the noun they describe.

teknik = technique
dasar = basic / fundamental

So:

teknik dasar = basic technique(s)
buku baru = new book
mobil merah = red car

This is the normal noun–adjective order in Indonesian: noun + adjective.

How can I clearly show that teknik is plural, like basic techniques?

Indonesian often leaves number (singular/plural) implicit. teknik dasar can mean a basic technique or basic techniques, depending on context.

To make plural more explicit, you can:

Repeat the noun:
teknik-teknik dasar = basic techniques (plural)
Use a quantifier or number:
beberapa teknik dasar = several basic techniques
tiga teknik dasar = three basic techniques

So:

Guru menjelaskan beberapa teknik dasar di kelas.
= The teacher explains several basic techniques in class.

Can I change the word order to Guru di kelas menjelaskan teknik dasar?

Yes, that is grammatically correct:

Guru menjelaskan teknik dasar di kelas. (more neutral: subject → verb → object → place)
Guru di kelas menjelaskan teknik dasar. (puts extra emphasis on guru di kelas, the teacher in that class)

The second version can sound like you are distinguishing this teacher from others (for example, the teacher in the class vs. the teacher in the lab). The neutral, most common order is the original sentence.

Do I need a pronoun like dia or beliau for he/she, or is guru enough?

You do not need a pronoun here. Indonesian often uses a noun (or name/title) as the subject without adding a separate pronoun.

Guru menjelaskan teknik dasar di kelas. = The teacher explains basic techniques in class.

If you want to refer back to the same teacher in the next sentence, you might then use:

Dia = he/she (neutral)
Beliau = he/she (polite, respectful, often for teachers, elders, officials)

Example:

Guru menjelaskan teknik dasar di kelas. Dia sangat sabar.
= The teacher explains basic techniques in class. He/She is very patient.

Is guru formal? How do students usually refer to a teacher?

Guru itself simply means teacher and is neutral.

Students typically address teachers using Pak (for men) or Bu (for women), often with a name:

Pak Budi (Mr. Budi, male teacher)
Bu Sari (Ms./Mrs. Sari, female teacher)
Sometimes: Pak Guru, Bu Guru (Mr./Mrs. Teacher)

In a sentence about a specific known teacher, you might say:

Pak Budi menjelaskan teknik dasar di kelas.
Bu Sari menjelaskan teknik dasar di kelas.

But as a general statement, just Guru menjelaskan teknik dasar di kelas is fine and natural.