Learn Hardcore German: Es ist unhöflich, im Kurs ständig laut zu reden. - It is impolite to talk loudly all the time in class.

es here is a dummy or placeholder “it” (like in English It is important to...).
The real action is in the zu + infinitive part: zu reden (to talk).
unhöflich is the adjective that comments on that action: impolite.

What is the es doing in Es ist unhöflich? Does it refer to anything?

No, es here does not refer to any specific thing. It is:

a dummy subject / formal subject, like in English:
- It is rude to interrupt.
- It is difficult to understand.

In German, you must put something in the subject position, so German uses es:

Es ist unhöflich, im Kurs ständig laut zu reden.
(It is impolite to talk loudly in class all the time.)

Why is there a comma before im Kurs ständig laut zu reden?

Because im Kurs ständig laut zu reden is an infinitive clause (zu reden + its extra information).

General rule:

An infinitive clause with zu can be separated from the main clause with a comma.
In many cases the comma is optional, but it is very commonly used and stylistically preferred.

Here:

Es ist unhöflich, ← main clause
im Kurs ständig laut zu reden. ← infinitive clause

So the comma marks the boundary between main clause and infinitive clause.

Why is it zu reden and not just reden?

In this construction, German needs zu before the infinitive verb:

Es ist unhöflich, … zu reden.
Es ist wichtig, pünktlich zu kommen.
Es ist schwer, das zu verstehen.

Compare:

English: to talk, to come, to understand
German: zu reden, zu kommen, zu verstehen

So zu here corresponds to English to in to + verb when used as a noun-like action (“to talk”, “to come” as activities).

What is the difference between reden, sprechen, and sagen? Why use reden here?

Rough meanings:

reden – to talk (more about general talking, often a bit informal)
sprechen – to speak (more neutral/formal; can be about languages or giving a speech)
sagen – to say (focus on what is said)

In this sentence:

laut reden = to talk loudly
The focus is on the activity of talking (making noise), not on the content.

We usually complain about someone laut zu reden or zu laut zu reden, not so much laut zu sprechen, in this “rude in class” context. So reden fits better with the idea of noisy, constant talking.

What exactly does ständig mean, and how is it different from immer or die ganze Zeit?

ständig = constantly, repeatedly, all the time
Often with a negative or annoyed tone:
- Er ist ständig zu spät. – He’s constantly late (and it’s annoying).

Compare:

immer = always (more neutral, can be factual)
die ganze Zeit = the whole time, all the time (descriptive, can also sound annoyed but is more literal)

In the sentence:

ständig laut zu reden = to talk loudly again and again / all the time
It suggests habitual, disturbing behavior in the course.

What does im Kurs mean, and why is it im and not in dem?

im is the contracted form of in dem:
- in (in) + dem (dative singular of der)
- in dem Kurs → im Kurs

German often shortens:

in dem → im
an dem → am
bei dem → beim, etc.

Kurs is masculine (der Kurs), and after in with a static meaning (in class as a location), you use dative:

in dem Kurs (dative masculine singular) → im Kurs

Why is the word order im Kurs ständig laut zu reden? Could it be different?

The infinitive group is:

im Kurs ständig laut zu reden

Normal order inside that phrase is:

Adverbial of place: im Kurs
Adverb of frequency: ständig
Adverb of manner: laut
Verb (infinitive): zu reden

Other orders are possible, but the original sounds natural and clear. For example:

ständig im Kurs laut zu reden – also possible
laut ständig im Kurs zu reden – possible but a bit unusual/emphatic

The zu must stay directly before the infinitive reden, and the whole group must stay together as the infinitive clause.

What does unhöflich literally mean, and is there a related positive form?

höflich = polite, courteous
un- = a common prefix meaning un- / not / in- (negation)

So:

unhöflich = impolite, rude, discourteous

You can pair them like:

höflich sein – to be polite
unhöflich sein – to be impolite

Could I also say Es ist unhöflich, wenn man im Kurs ständig laut redet? What’s the difference?

Yes, that is correct:

Es ist unhöflich, wenn man im Kurs ständig laut redet.
= It is impolite when one talks loudly all the time in class.

Difference in feel:

Es ist unhöflich, im Kurs ständig laut zu reden.
→ more compact, abstract; focuses on the action as a general behavior.
Es ist unhöflich, wenn man im Kurs ständig laut redet.
→ more like a condition / situation: It is rude if/when someone does that.

Both express almost the same idea; the zu + infinitive version is just a little more concise and “grammatical” sounding, while the wenn man … version feels more concrete and narrative.