Learn Hardcore German: Im Kurs nennen alle dieses Wort schwierig. - In the course everyone calls this word difficult.

alle = they all / everyone (in the course) → this is who is doing the action.
nennen is conjugated for 3rd person plural (sie nennen), which matches alle.
dieses Wort is the direct object (accusative) = this word.
schwierig is a predicative adjective describing the object dieses Wort.

Structure (simplified):

Im Kurs – adverbial (where?)
alle – subject
nennen – verb
dieses Wort – direct object (accusative)
schwierig – adjective complement to the object

So the core meaning is: Alle nennen dieses Wort schwierig = They all call this word difficult.

Why does the sentence start with Im Kurs? Could we also say Alle nennen im Kurs dieses Wort schwierig?

German has a verb-second rule: in a main clause, the conjugated verb must be the second element in the sentence.

You can put different elements in the first position to change the focus/topic. Here, Im Kurs (in the course) is put first to set the scene:

Im Kurs nennen alle dieses Wort schwierig.
→ Emphasis: in the course (as opposed to outside it).

Other correct word orders (with slightly different emphasis) include:

Alle nennen dieses Wort im Kurs schwierig.
Dieses Wort nennen alle im Kurs schwierig.
Alle nennen im Kurs dieses Wort schwierig. (less neutral, but possible)

In all of these, the conjugated verb nennen stays in second position:

[Element X]
nennen
the rest of the sentence

Starting with Im Kurs is just a stylistic choice to foreground the context.

What does Im Kurs mean exactly, and why im and not in dem?

Im is simply the contracted form of in dem:

in (preposition) + dem (dative form of der) → im

Kurs is masculine: der Kurs.
With the preposition in used in a static / location sense (where something happens), German uses the dative case:

in dem Kurs (dative singular masculine) → contracted to im Kurs

So:

In dem Kurs nennen alle dieses Wort schwierig.
Im Kurs nennen alle dieses Wort schwierig.

mean the same, but im Kurs is what you will actually hear and read in normal German.

Why is it dieses Wort and not dieser Wort or diesem Wort?

Wort is a neuter noun:

das Wort (nominative singular)

In the sentence, dieses Wort is the direct object (what is being called difficult), so it is in the accusative case.

For a neuter noun, the demonstrative dies- looks like this:

Nominative neuter: dieses Wort
Accusative neuter: dieses Wort

So nominative and accusative neuter are the same form: dieses.

Compare:

dieses Wort ist schwierig – this word is difficult (subject, nominative)
Alle nennen dieses Wort schwierig – they all call this word difficult (object, accusative)

dieser Wort or diesem Wort would be wrong because those endings are for other gender/case combinations (e.g. masculine nominative, dative, etc.).

Why is schwierig without an ending? Why not schwieriges?

In German, adjectives behave differently depending on their role:

Attributive (in front of a noun) → with endings
- ein schwieriges Wort – a difficult word
- das schwierige Wort – the difficult word
Predicative (after certain verbs like sein, werden, bleiben, and in patterns like etwas adj. nennen / finden) → no endings
- Das Wort ist schwierig. – The word is difficult.
- Alle finden das Wort schwierig. – Everyone finds the word difficult.
- Alle nennen dieses Wort schwierig. – Everyone calls this word difficult.

In your sentence, schwierig is not directly attached to a noun; it’s describing the object dieses Wort through the verb nennen. That’s why it appears in its base form, without an ending.

How exactly is nennen being used here? Does it mean “name” or “call”?

nennen can mean:

to name / to give a name
- Wir nennen den Hund Max. – We call/name the dog Max.
to call / to refer to something as (Adj.) (the pattern in your sentence)
- Structure: jemanden / etwas + Adj. nennen
- Meaning: to call / describe / label someone or something as (Adj.)

So:

Alle nennen dieses Wort schwierig.
= Everyone calls this word difficult.
= Everyone considers / labels this word as difficult.

This is similar to:

Alle finden dieses Wort schwierig. – Everyone finds/considers this word difficult.
Alle halten dieses Wort für schwierig. – Everyone regards this word as difficult.

But nennen here feels a bit like: “They (habitually) refer to this word as ‘difficult’.” It has a slight nuance of how people talk about it.

Could we also say Im Kurs finden alle dieses Wort schwierig or Im Kurs ist dieses Wort schwierig?

Im Kurs finden alle dieses Wort schwierig.
This is perfectly correct and very natural.
Meaning: In the course, everyone finds this word difficult.
It focuses more on people’s opinion or impression.
Im Kurs ist dieses Wort schwierig.
This is grammatically possible, but sounds odd in most contexts.
Literally: In the course, this word is difficult.
It suggests that the word itself has some property of “being difficult” specifically in that course, which is a strange way to say it.

If you want to express what the original sentence expresses, the best alternatives are:

Im Kurs nennen alle dieses Wort schwierig.
Im Kurs finden alle dieses Wort schwierig.
Im Kurs halten alle dieses Wort für schwierig.

All three describe that people judge the word to be difficult.

Why is it alle and not jeder? What is the difference between alle and jeder here?

Both are possible, but they behave differently:

alle = all (of them), plural
- Im Kurs nennen alle dieses Wort schwierig.
  → Literally: In the course, all (people) call this word difficult.
jeder = each / every (one), singular
- Im Kurs nennt jeder dieses Wort schwierig.
  → Literally: In the course, each (person) calls this word difficult.

Differences:

alle suggests the group as a whole: everyone together.
jeder looks more at each individual in the group.

Also note verb agreement:

alle → plural verb: nennen
jeder → singular verb: nennt

What other word orders are possible and still correct for this sentence?

As long as the conjugated verb (nennen) stays in second position, you can move the other parts around. Some natural versions:

Im Kurs nennen alle dieses Wort schwierig.
Alle nennen in dem Kurs dieses Wort schwierig.
Alle nennen dieses Wort im Kurs schwierig.
Dieses Wort nennen alle im Kurs schwierig.
Dieses Wort nennen im Kurs alle schwierig. (a bit marked, but possible)

Less natural or wrong examples:

✗ Im Kurs alle nennen dieses Wort schwierig.
→ Here nennen is not in second position. Incorrect.
✗ Alle dieses Wort nennen im Kurs schwierig.
→ Again, nennen is not second. Incorrect.

So: you can shuffle Im Kurs / Alle / dieses Wort around for emphasis, but nennen must remain the second sentence element.

How would the sentence change if I replace dieses Wort with a pronoun like es? Where does es go?

If the context already makes das Wort clear, Germans often replace dieses Wort with es:

Common natural patterns:

Alle nennen es im Kurs schwierig.
Im Kurs nennen alle es schwierig. (possible, but less natural than the next one)
Im Kurs nennen es alle schwierig. (also possible; emphasis slightly shifted)

A very typical neutral version would be:

Alle nennen es schwierig.
Im Kurs nennen alle es schwierig. / Im Kurs nennen alle es schwierig is grammatically fine, but many speakers prefer to put the short pronoun es as early as possible:
→ Alle nennen es im Kurs schwierig.

Rule of thumb:
Short pronouns (es, ihn, sie, ihn) tend to appear earlier in the middle field, often right after the conjugated verb, unless you move something in front for emphasis.

Why is it im Kurs and not something like im Unterricht? Do they mean the same thing?

They are related, but not identical:

der Kurs = a course, usually a specific class with a defined group, level, and duration.
- Im Kurs = in the course (as a group / in this class context)
der Unterricht = instruction / teaching / lessons in a more general sense.
- Im Unterricht = during class / during lessons

So:

Im Kurs nennen alle dieses Wort schwierig.
→ Within this particular course / among the people enrolled in it, everyone calls this word difficult.
Im Unterricht nennen alle dieses Wort schwierig.
→ During the lessons / while class is going on, everyone calls this word difficult.

Often they will overlap in real life, but Kurs emphasizes the course as a group/program, while Unterricht emphasizes the teaching situation. Both are grammatical; which one you choose depends on what you want to highlight.