Det er situationen, der gør hende nervøs.

This is a very common Danish emphasis pattern, often called a cleft sentence.

• Situationen gør hende nervøs = The situation makes her nervous
• Det er situationen, der gør hende nervøs = It is the situation that makes her nervous

The second version puts special focus on situationen. It suggests something like:

• It’s the situation, not something else
• What makes her nervous is the situation

So the sentence does not just state a fact; it highlights what the cause is.

Because situationen is the definite singular form of the noun.

• en situation = a situation
• situationen = the situation

In Danish, the definite article is often added to the end of the noun as a suffix.

Since the sentence means the situation, Danish uses situationen.

The comma separates the main clause from the relative clause.

The structure is:

• Det er situationen = main clause
• der gør hende nervøs = relative clause

The relative clause describes situationen more closely: it is the situation that makes her nervous.

Modern Danish punctuation normally uses a comma before a relative clause like this.

Here, der means that/who/which in a relative clause, but more specifically it is used because it is the subject of the relative clause.

In:

• der gør hende nervøs

the word der stands for situationen, and situationen is the thing doing the action of gør.

So:

• situationen gør hende nervøs
• in the relative clause, situationen becomes der

This is very common in Danish after det er ... cleft sentences:

• Det er Peter, der kommer først
• Det er bilen, der er dyr
• Det er situationen, der gør hende nervøs

In this kind of sentence, der is the normal choice.

A useful rule is:

• Use der when the relative word is the subject of the relative clause.
• Use som in many other relative clauses, especially when it is not the subject.

Here, in der gør hende nervøs, the relative word is the subject of gør, so der is preferred.

Also, after the cleft pattern Det er X, der ..., Danish strongly prefers der.

So Det er situationen, der gør hende nervøs sounds natural and standard.

Because hende is the object form of hun.

• hun = she (subject form)
• hende = her (object form)

In this sentence, the subject is situationen / der, because that is what is doing the action:

• situationen gør hende nervøs

The person being affected is her, so Danish uses hende.

Compare:

• Hun er nervøs = She is nervous
• Situationen gør hende nervøs = The situation makes her nervous

This is a very common Danish pattern:

gøre + object + adjective

It means make someone/something + adjective.

So:

• gør hende nervøs = makes her nervous

Other examples:

• Det gør mig glad = It makes me happy
• Nyheden gjorde ham trist = The news made him sad
• Støjen gør dem irriterede = The noise makes them irritated

So gøre is not just do here. It can also mean make/cause someone to become a certain way.

Because nervøs is used as a predicative adjective here, not as an adjective directly in front of a noun.

Compare:

• en nervøs kvinde = a nervous woman
• hun er nervøs = she is nervous
• det gør hende nervøs = it makes her nervous

In the last two examples, nervøs functions like a complement, and it normally stays in its basic form.

So you do not say nervøse or anything else here.

Yes, mostly in emphasis, not in basic meaning.

• Situationen gør hende nervøs = neutral statement
• Det er situationen, der gør hende nervøs = focused/emphatic statement

The second one often implies contrast, for example:

• Not the people
• Not the place
• Not the task
• The situation

So both can mean the same thing in a broad sense, but the Det er ... der ... version highlights the cause more strongly.

Yes and no.

In form, det is the Danish word for it, but in this structure it mainly acts as part of the fixed cleft pattern:

• Det er X, der ...

This pattern is extremely common in Danish and often does not refer to a specific thing called it in the same way English learners might expect.

So it is best to learn Det er ... der ... as a whole structure meaning something like:

• It is X that ...
• What ... is X

Not perfectly, because the structure is more important than a literal word-for-word match.

A rough breakdown is:

• Det = it
• er = is
• situationen = the situation
• der = that
• gør = makes
• hende = her
• nervøs = nervous

So a very literal version is:

• It is the situation that makes her nervous

That works well in English too, but in real translation you should focus on the function of the sentence, especially the emphasis.

Gør is pronounced roughly like a short, rounded vowel followed by a soft r sound. For many English speakers, it can feel unfamiliar because Danish ø does not exist in standard English.

A few helpful points:

• ø is a front rounded vowel
• the r in Danish is not pronounced like a strong English r
• the word is short and compact

This is one of those words where listening and repeating is much more useful than trying to force it into English spelling. If you are learning pronunciation, it is worth hearing native audio for gør, because it appears very often in Danish.