Learn Hardcore Danish: En hypotetisk sætning fortæller, hvad der ville ske, hvis noget var anderledes. - A hypothetical sentence tells what would happen if something were different.

You just have to learn the gender with each noun; there’s no reliable rule from meaning or spelling. Since sætning is an en-word, you must say en hypotetisk sætning.

What kind of word is hypotetisk, and does it change form?

Hypotetisk is an adjective meaning hypothetical.

In this sentence it stands in front of a noun: en hypotetisk sætning.

About its forms:

Singular common gender (en-word): en hypotetisk sætning
Singular neuter (et-word): et hypotetisk eksempel
Plural: hypotetiske sætninger / hypotetiske eksempler

So:

It does not add -t in the neuter (because adjectives ending in -isk are special).
It does add -e in the plural: hypotetiske.

Why is the verb fortæller used, and what form is it?

Fortæller is the present tense of at fortælle (to tell).

(en sætning) fortæller = (a sentence) tells

It’s used here because the sentence describes a general truth or definition:

En hypotetisk sætning fortæller …
= A hypothetical sentence tells … (in general, whenever you have such a sentence)

In Danish, just like in English, you use the present tense for definitions and general statements.

What is the structure and function of hvad der ville ske?

Hvad der ville ske is a subordinate clause (an object clause) meaning what would happen.

Breakdown:

hvad = what
der = dummy subject (“there”)
ville = would
ske = happen

In English word order:
hvad der ville ske ≈ what there would happen → idiomatically: what would happen.

Why der?
In Danish embedded clauses like this, you often use der as a formal (dummy) subject, similar to English dummy “there”:

Fortæl mig, hvad der skete.
= Tell me what (there) happened.

Without der, hvad ville ske is possible in some contexts, but hvad der ville ske is the most natural, standard form here.

Why is it ville ske and not just skete or vil ske?

Because the sentence describes a hypothetical situation, so Danish uses ville to express conditional “would”:

ville ske = would happen

Contrast:

hvad der skete = what happened (simple past, factual)
hvad der vil ske = what will happen (future, expected)
hvad der ville ske = what would happen (hypothetical / imagined)

Since the sentence is about hypothetical sentences, ville ske matches the meaning “would happen if things were different.”

Why is there a comma before hvad and before hvis?

Because Danish usually sets commas before subordinate clauses.

The structure is:

Main clause:
En hypotetisk sætning fortæller, …
Object clause (subordinate):
… hvad der ville ske, …
Conditional clause (subordinate):
… hvis noget var anderledes.

So we get:

En hypotetisk sætning fortæller, / hvad der ville ske, / hvis noget var anderledes.

Modern rules allow a little variation (some people might drop the first comma), but having commas before hvad and hvis like this is very common and fully correct.

Why is var (past tense) used in hvis noget var anderledes if we’re talking about a hypothetical situation, not the past?

Danish uses the past tense in if-clauses to talk about unreal / hypothetical situations, just like English:

Hvis noget var anderledes
= If something were different (not actually so)

So:

Hvis noget er anderledes = If something is different (real / possible, present)
Hvis noget var anderledes = If something were different (unreal / imagined)

This is called a “past subjunctive” type use: the form is past, but the meaning is hypothetical, not necessarily past in time.

What does noget mean here, and how is it used?

Noget is an indefinite pronoun meaning something (or anything, depending on context).

In this sentence:

noget var anderledes = something was different

Typical uses of noget:

Jeg vil have noget at spise. = I want something to eat.
Er der noget galt? = Is something wrong?

Notice that noget can also appear with adjectives:

noget anderledes = somewhat different / something different (depending on context)

Here, noget is the subject of the clause: something would be different.

What is the difference between hvis and når, and why is hvis used here?

hvis = if (condition, often hypothetical or uncertain)
når = when (time, something you expect to happen regularly or certainly)

Examples:

Hvis det regner, bliver jeg hjemme.
= If it rains, I’ll stay home. (maybe it will, maybe not)
Når det regner, bliver jeg hjemme.
= When it rains, I stay home. (whenever that happens)

In hvis noget var anderledes, the situation is hypothetical / unreal, so hvis (if) is the correct choice, not når.

What kind of word is anderledes, and why doesn’t it change form?

Anderledes means different and can function as both an adjective and an adverb, but it is invariable – it doesn’t change form:

Han er anderledes. = He is different.
De er anderledes. = They are different.
Hun tænker anderledes. = She thinks differently.

Compare with a regular adjective like stor (big):

en stor bil (sing., common)
et stort hus (sing., neuter)
store biler / huse (plural)

Anderledes does not do this:

et anderledes hus (not anderledest or anderledese)

So in noget var anderledes, anderledes stays in its base form.

Could you rephrase the Danish sentence with a different word order, for example starting with the hvis-clause?

Yes, you can move the hvis-clause to the front, but you must adjust the word order in the main clause that follows:

Original:
En hypotetisk sætning fortæller, hvad der ville ske, hvis noget var anderledes.
Alternative:
Hvis noget var anderledes, ville en hypotetisk sætning fortælle, hvad der ville ske.

Notice:

The hvis-clause comes first.
In the following main clause, the verb (ville) must come right after the clause, in second position (V2 word order):
Hvis noget var anderledes, ville en hypotetisk sætning fortælle …

What is the difference between fortæller and siger? Could we say En hypotetisk sætning siger …?

Both fortæller and siger can mean tells / says, but there is a nuance:

at sige = to say (more neutral, basic reporting of words)
at fortælle = to tell (often implies relating, explaining, giving information or a story)

For definitions or explanations, fortælle fits very well, because a sentence is telling you something about a situation.

You could say:

En hypotetisk sætning siger, hvad der ville ske, hvis noget var anderledes.

This is understandable, but fortæller sounds a bit more natural and descriptive when talking about what a sentence conveys or explains.