Learn Hardcore Malay: Di kelas, guru menggalakkan murid berhujah dengan sopan supaya perbincangan adalah adil. - In class, the teacher encourages students to argue politely so that the discussion is fair.

Di is a preposition meaning at / in / on (location).
Di kelas = in class / in the classroom (more general: the setting is “class time / in class”).

You can say dalam kelas, which literally means inside the classroom and focuses more on being physically inside the room.

Di kelas – neutral, very common for “in class / during class”.
Dalam kelas – a bit more spatial/physical: “inside the classroom”.

Both are acceptable here; di kelas is slightly more natural for the idea of a class setting or lesson time.

Why is there no word for the or a before guru and murid?

Malay generally does not use articles like the or a/an.

So:

guru can mean a teacher, the teacher, or teachers, depending on context.
murid can mean a pupil, the pupils, or pupils/students in general.

Context supplies what English would express with a/the/some.
In this sentence, guru is understood as the teacher, and murid as the pupils/students in that class.

How is menggalakkan formed, and why not just galak?

The base word is galak, which (in this context) relates to being eager/encouraging.

Malay commonly forms verbs with prefixes and suffixes:

meN- + galak + -kan → menggalakkan

Details:

meN- is an active verb prefix ("to do X").
-kan often makes the verb transitive, taking an object.
With a root starting with g, meN- becomes meng- (phonological rule).

So:

menggalakkan = to encourage (someone to do something).
Guru menggalakkan murid... = The teacher encourages the students...

Using just galak here would be wrong; you need the proper verb form menggalakkan to mean encourages.

What is the difference between murid and pelajar?

Both can be translated as student, but there is a nuance:

murid – more often used for school pupils, especially primary and secondary school.
pelajar – more general student, often for secondary school and above, especially college/university.

In this sentence, murid suggests school pupils rather than university students, though in informal speech people sometimes use them interchangeably.

Why is the verb berhujah and not menghujah? What does ber- do here?

The root is hujah = argument (as a noun).

Malay often uses ber- to make intransitive verbs that mean “to have/do X”:

berjalan – to walk (to go in a walking way)
berbincang – to discuss
bermain – to play
berhujah – to argue / to engage in argument

So berhujah means to argue / to debate (in general).

Menghujah exists but is much less common and more like to argue (a case) / to present arguments about something in a more formal/legal sense.
For “students argue/debate”, berhujah is the natural choice.

What does dengan sopan literally mean, and could I use something else for “politely”?

Literally:

dengan = with / in a ... way
sopan = polite

So berhujah dengan sopan = to argue with politeness / to argue in a polite way.

You could also say:

berhujah secara sopan – using secara (“in a ... manner/style”), a bit more formal.
berhujah dengan sopan santun – “with politeness and good manners”, slightly stronger.

But dengan sopan is simple, natural, and very common.

What does supaya mean, and how is it different from untuk?

Supaya is a conjunction meaning so that / in order that, introducing a clause with a subject and verb:

supaya perbincangan adalah adil
= so that the discussion is fair

Untuk means for / in order to, and is usually followed by a verb phrase or noun, not a full clause with its own subject:

untuk berlaku adil – in order to be fair
untuk perbincangan yang adil – for a fair discussion

So:

Use supaya before a full clause: supaya + [subject + verb].
Use untuk before a verb (infinitive-like) or noun: untuk + [verb/noun].

In this sentence, supaya is correct because perbincangan adalah adil is a full clause.

Is adalah necessary in supaya perbincangan adalah adil? Could we omit it?

You can omit adalah here:

supaya perbincangan adil – also correct and natural.

Adalah is a kind of linking verb often used in more formal writing, especially before nouns/noun phrases:

perbincangan itu adalah satu proses...

Before a simple adjective like adil, many style guides actually prefer omitting adalah:

Perbincangan itu adil. (preferred in standard style)
Perbincangan itu adalah adil. (used, sounds more formal/bookish; some consider it less standard)

So in everyday and even standard usage, supaya perbincangan adil is perfectly fine and arguably more natural.

Why is there no word like to before berhujah, as in “encourages (them) to argue”?

Malay does not always need a separate word like to (English infinitive marker).
A common pattern is:

[Subject] + menggalakkan + [object] + [verb]
Guru menggalakkan murid berhujah.
= The teacher encourages the students to argue.

You can insert untuk:

Guru menggalakkan murid untuk berhujah dengan sopan.

Both are acceptable. Without untuk, the sentence is shorter and still clear; berhujah is understood as what the murid are encouraged to do.

How do tense and time work here? How do we know if this is present or past?

Malay normally does not change the verb form for tense.
Menggalakkan, berhujah, adalah all stay the same for past, present, or future.

The sentence can mean:

In class, the teacher encourages the students... (habitual present)
In class, the teacher encouraged the students... (past)
In class, the teacher will encourage the students... (future, from context)

If you really need to specify time, you add time words:

semalam – yesterday
setiap hari – every day
nanti – later / in the future

Example:

Semalam di kelas, guru menggalakkan murid berhujah dengan sopan.
= Yesterday in class, the teacher encouraged the students to argue politely.

Why perbincangan and not bincang or berbincang?

Root: bincang = to discuss (basic idea).

Malay uses different affixes to change word class:

berbincang – verb: to discuss / to have a discussion
(e.g. Mereka berbincang. – They are discussing.)
perbincangan – noun: discussion / the act of discussing
peN- + bincang + -an → perbincangan

In this sentence, you need a noun after supaya:

supaya perbincangan adalah adil
= so that the discussion is fair

Using bincang or berbincang here would be ungrammatical; they are not nouns.