Wenn du Fieber hast, musst du gleich zum Arzt gehen.

Because wenn introduces a subordinate clause. In German, the finite verb usually goes to the end of a subordinate clause.

• wenn du Fieber hast = if/when you have a fever
• normal main-clause order would be: du hast Fieber
• after wenn, it becomes: wenn du Fieber hast

This is one of the most important word-order rules in German.

Because the sentence starts with the wenn-clause, and that entire clause takes up the first position in the sentence. In a German main clause, the finite verb must be in second position.

So the structure is:

• Wenn du Fieber hast, = position 1
• musst = position 2
• du = then the subject

That is why you get:

• Wenn du Fieber hast, musst du ...

If you start with the main clause instead, you can say:

• Du musst gleich zum Arzt gehen, wenn du Fieber hast.

Here, wenn means if.

So:

• Wenn du Fieber hast ... = If you have a fever ...

German wenn can also mean when in some contexts, especially for repeated or general situations:

• Wenn ich krank bin, bleibe ich zu Hause. = When/Whenever I am sick, I stay at home.

Useful contrast:

• wenn = if, or when/whenever for repeated situations
• als = when for a single event in the past
• wann = when? in a question

Examples:

• Wenn du Zeit hast, komm vorbei. = If you have time, come by.
• Als ich krank war, ... = When I was sick ...
• Wann gehst du zum Arzt? = When are you going to the doctor?

In German, a subordinate clause is normally separated from the main clause by a comma.

So in:

• Wenn du Fieber hast, musst du gleich zum Arzt gehen.

the comma is required because Wenn du Fieber hast is a subordinate clause.

This is much stricter than in English, where comma use can be more flexible.

German commonly uses haben with certain illnesses or symptoms:

• Fieber haben = to have a fever
• Kopfschmerzen haben = to have a headache
• Halsschmerzen haben = to have a sore throat

So du hast Fieber is the normal, idiomatic way to say you have a fever.

You can also say:

• du bist fiebrig = you are feverish

But Fieber haben is more common in everyday speech.

musst is from müssen, which usually means must, have to, or need to, depending on context.

Here it means strong necessity:

• du musst ... gehen = you must / have to go

In everyday English, have to is often the most natural translation.
So this sentence often feels like:

• If you have a fever, you have to go to the doctor right away.

German müssen is very common and often sounds less dramatic than English must can sometimes sound.

Because musst is a modal verb. With modal verbs in German, the second verb usually appears in the infinitive at the end of the clause.

Structure:

• du musst gehen
• you must go

In this sentence:

• musst = finite modal verb
• gehen = infinitive at the end

So:

• musst du gleich zum Arzt gehen

This is normal German word order with modal verbs.

Here, gleich means right away, immediately, or at once.

So:

• gleich zum Arzt gehen = go to the doctor right away

This is not the equal/same meaning of gleich. German gleich can have several meanings depending on context:

• Das ist gleich. = That is the same/equal.
• Ich komme gleich. = I’m coming right away.

In your sentence, it clearly means without delay.

zum is just the common contraction of:

• zu dem → zum

So:

• zum Arzt = to the doctor

This contraction is extremely common and usually preferred in normal speech and writing.

The preposition zu takes the dative case.

• der Arzt = nominative
• dem Arzt = dative
• zu dem Arzt = zum Arzt

So Arzt is in the dative singular here.

Also, zu is the normal preposition for going to a person or to a professional:

• zum Arzt = to the doctor
• zum Lehrer = to the teacher
• zum Friseur = to the hairdresser

You would not say in den Arzt, because in is not the right preposition for going to a doctor as a person.

Yes. du is the informal singular form of you.

The formal version would use Sie:

• Wenn Sie Fieber haben, müssen Sie gleich zum Arzt gehen.

Changes to notice:

• du → Sie
• hast → haben
• musst → müssen

Also remember that formal Sie is always capitalized.

Yes. You can also say:

• Du musst gleich zum Arzt gehen, wenn du Fieber hast.

That means the same thing.

The difference is mostly about focus and style:

• Wenn du Fieber hast, musst du ... puts the condition first
• Du musst ..., wenn du Fieber hast starts with the main message

Both are correct.

Arzt is the masculine form. The feminine form is Ärztin.

So you could say:

• zum Arzt = to the doctor / to a male doctor
• zur Ärztin = to the female doctor

Note the contraction:

• zu der → zur

So:

• Wenn du Fieber hast, musst du gleich zur Ärztin gehen.

In many contexts, though, zum Arzt can be used generally in a traditional way to mean to the doctor without focusing on gender. Still, modern German often uses more gender-specific wording when needed.

Because the verb ending tells you the subject is du.

• hast = you have
• so du is the subject

In du Fieber hast:

• du = subject
• Fieber = object

This is similar to English:

• you have a fever

The person has the fever, so the person is the subject, not the fever.