Der Arzt, den ich heute sehe, ist sehr freundlich.

Because den is the relative pronoun in the accusative masculine form.

The noun being described is der Arzt, which is masculine singular. In the relative clause, the doctor is the object of sehen:

• ich = the subject, the person doing the seeing
• den = the object, the person being seen

So German uses:

• der Arzt in the main clause because it is the subject there
• den in the relative clause because it is the direct object there

A very literal breakdown is:

• Der Arzt = the doctor
• den ich heute sehe = whom I am seeing today

So den is correct because the verb sehen takes an accusative object.

You know from the case marking.

In den ich heute sehe:

• ich is nominative, so it is the subject
• den is accusative, so it is the object

That tells you:

• I am seeing the doctor

not:

• the doctor is seeing me

This is one of the big advantages of German case forms: they often make the grammatical roles clearer than English does.

Because den ich heute sehe is a relative clause, and relative clauses are a type of subordinate clause in German.

In German subordinate clauses, the conjugated verb usually goes to the end.

So instead of the main-clause order:

• Ich sehe den Arzt heute.

you get the relative clause order:

• den ich heute sehe

The pattern is:

• relative pronoun
• subject / other elements
• verb at the end

That is very typical German subordinate-clause word order.

Because German normally sets off a relative clause with commas.

The relative clause here is:

• den ich heute sehe

It gives extra information about der Arzt, so it is separated with commas:

• Der Arzt, den ich heute sehe, ist sehr freundlich.

This is standard German punctuation. In English, relative clauses are not always marked exactly the same way, so this often stands out to learners.

Here, den is not the normal article in the sentence structure. It is a relative pronoun.

It looks identical to the masculine accusative form of the, but its job is different. It means something like:

• whom
• or more naturally in English, often just that or nothing at all

So:

• der Arzt = the doctor
• den ich heute sehe = whom I am seeing today

German relative pronouns often have the same forms as definite articles, which is why this can be confusing at first.

Because the part before ist is all one unit: the subject phrase.

The main clause is:

• Der Arzt ... ist sehr freundlich.

The subject is not just Der Arzt by itself, but:

• Der Arzt, den ich heute sehe,

Then the verb of the main clause, ist, comes in the usual main-clause position.

So the sentence has:

• main clause subject: Der Arzt, den ich heute sehe
• main clause verb: ist
• complement: sehr freundlich

The relative clause interrupts the subject phrase, but it does not change the basic structure of the main clause.

Because freundlich is a predicate adjective here, not an adjective directly before a noun.

After verbs like:

• sein = to be
• werden = to become
• bleiben = to stay

German adjectives usually do not take adjective endings.

So:

• Der Arzt ist freundlich. = The doctor is friendly.

But before a noun, the adjective does take an ending:

• ein freundlicher Arzt = a friendly doctor
• der freundliche Arzt = the friendly doctor

So in your sentence, freundlich stays in its basic form.

Yes, heute can move, but the sentence given is very natural.

In the relative clause:

• den ich heute sehe

heute comes before the verb, which is normal because the verb must stay at the end.

You may also hear slightly different word orders depending on emphasis, for example:

• Der Arzt, den ich sehe, ist sehr freundlich.
• Der Arzt, den ich heute sehe, ist sehr freundlich.

Within the subordinate clause, heute is placed before sehe, and that sounds natural and standard.

Yes. Grammatically, den corresponds most closely to whom, because it is the object of the verb.

So a very exact translation would be:

• The doctor, whom I am seeing today, is very friendly.

But in natural English, many people would say:

• The doctor who I’m seeing today is very friendly.
• The doctor that I’m seeing today is very friendly.
• The doctor I’m seeing today is very friendly.

German keeps the case distinction clearly with den, while English often does not.

Yes, that is a very good way to think about it.

The core sentence is:

• Der Arzt ist sehr freundlich. = The doctor is very friendly.

Then German inserts a relative clause to identify which doctor:

• den ich heute sehe = the one I’m seeing today

So the full sentence means:

• The doctor I’m seeing today is very friendly.

This is a useful strategy when reading German: first find the main clause, then treat the relative clause as descriptive information attached to a noun.