Der Arzt sagt, ich soll mein Knie noch zwei Tage schonen.

Because der Arzt is the subject of the first clause: Der Arzt sagt = The doctor says.

In German, the subject usually takes the nominative case. For the masculine noun Arzt, the nominative singular article is der.

• der Arzt = the doctor (subject / nominative)
• den Arzt = the doctor (direct object / accusative)
• dem Arzt = to the doctor (indirect object / dative)

So here, the doctor is the person doing the saying, which is why der Arzt is correct.

German normally uses a comma to separate clauses more often than English does.

Here you have:

• Der Arzt sagt
• ich soll mein Knie noch zwei Tage schonen

The second part gives the content of what the doctor says, so German separates it with a comma.

You can think of it like:

• Der Arzt sagt, dass ich mein Knie noch zwei Tage schonen soll.

In the original sentence, dass is omitted, but the comma still stays.

Ich soll is present tense of sollen, and here it means something like:

• I’m supposed to
• I should
• I am to

It often reports someone else’s instruction, advice, or expectation. In this sentence, it sounds like the doctor’s recommendation or order is being reported.

Ich sollte is more like:

• I should / ought to in a softer, more hypothetical, or less direct sense

So:

• Der Arzt sagt, ich soll ... = The doctor says I’m supposed to ...
• Der Arzt sagt, ich sollte ... = The doctor says I should ... (a bit softer or more tentative)

Here, soll expresses reported advice/instruction from the doctor.

So ich soll mein Knie schonen means:

• I’m supposed to rest/protect my knee
• The doctor says I should take it easy on my knee

This use of sollen is very common when you are passing on what someone else says:

• Er sagt, ich soll kommen. = He says I should come.
• Meine Mutter sagt, ich soll mehr schlafen. = My mother says I should sleep more.

So the force is not just general obligation; it is specifically what someone says you should do.

Because soll is a modal verb, and in German modal verbs usually send the main verb to the end in its infinitive form.

So:

• ich soll ... schonen

This is the normal pattern:

• Ich will gehen. = I want to go.
• Du musst lernen. = You must study.
• Wir können warten. = We can wait.

Here:

• soll = the conjugated modal verb
• schonen = the infinitive at the end

Schonen means to go easy on, to rest, to spare, or to avoid straining something.

With body parts, it often means:

• don’t put too much weight on it
• don’t overuse it
• let it recover

So mein Knie schonen is not literally just protect my knee in a dramatic sense; it means something more like:

• take it easy on my knee
• rest my knee
• avoid straining my knee

Because mein Knie is the direct object of schonen, and schonen takes the accusative case.

The noun Knie is neuter (das Knie), so:

• nominative: mein Knie
• accusative: mein Knie
• dative: meinem Knie

Since the form of neuter nouns is the same in nominative and accusative, you see mein Knie here.

Compare:

• Ich schone mein Knie. = I’m resting my knee. (accusative)
• Ich schade meinem Knie. = I’m harming my knee. (dative with a different verb)

Because all nouns in German are capitalized.

So:

• der Arzt
• das Knie
• der Tag

This is one of the most noticeable spelling differences from English.

Noch zwei Tage means for two more days.

• zwei Tage = two days
• noch here adds the idea of still / a further / an additional

So:

• noch zwei Tage = another two days / two more days

Examples:

• Ich bleibe noch zwei Tage. = I’m staying two more days.
• Warte noch fünf Minuten. = Wait another five minutes.

In your sentence, it means the knee should be rested for an additional two days from now.

German often uses an accusative expression of duration without a preposition.

So zwei Tage by itself can mean:

• for two days

That is very common.

Examples:

• Ich arbeite drei Stunden. = I work for three hours.
• Er bleibt eine Woche. = He’s staying for a week.
• Sie muss noch zwei Tage im Bett bleiben. = She has to stay in bed for two more days.

So noch zwei Tage works naturally without für.

Yes. That version is completely correct and means the same thing.

There are two common ways to say it:

1. Der Arzt sagt, ich soll mein Knie noch zwei Tage schonen.
2. Der Arzt sagt, dass ich mein Knie noch zwei Tage schonen soll.

The difference is mainly in structure:

• without dass, the second clause keeps normal word order: ich soll ...
• with dass, the verb goes to the end: ... schonen soll

Both are natural. The version without dass is very common in everyday German.

Both can be possible depending on emphasis, but ich soll mein Knie noch zwei Tage schonen is a very natural neutral order.

German word order is flexible, but it still has common patterns. Here:

• ich = subject
• soll = finite modal verb
• mein Knie = object
• noch zwei Tage = time expression
• schonen = infinitive at the end

Putting mein Knie before noch zwei Tage sounds very normal because the object is short and closely connected to schonen.

If you say:

• ich soll noch zwei Tage mein Knie schonen

that can also work, but it may put slightly more focus on the time span noch zwei Tage.

In traditional forms:

• der Arzt = male doctor
• die Ärztin = female doctor

So if the doctor is a woman, you would usually say:

• Die Ärztin sagt, ich soll mein Knie noch zwei Tage schonen.

The rest of the sentence stays the same.