Learn Hardcore Turkish: Arkadaşım, senin ne kadar nazik olduğunu söyledi.

The verb söyledi is 3rd person singular (he/she/it said), so it agrees with arkadaşım.
If arkadaşım were just a form of address (like My friend, how are you?), you would normally expect the verb to be 2nd person (söyledin, you said) when speaking to that friend.

So the sentence means: My friend said how kind you are, not My friend, you said how kind you are.

The comma after arkadaşım is not essential; you will also see:
Arkadaşım senin ne kadar nazik olduğunu söyledi.

Why is it arkadaşım and not benim arkadaşım for my friend?

Turkish usually shows possession with a suffix, and the separate pronoun is often dropped when it’s obvious.

arkadaş = friend
arkadaş-ım = my friend (-ım = my)

You can say benim arkadaşım, but that often adds emphasis:

Benim arkadaşım = my friend (as opposed to someone else’s)

In a neutral sentence like this, arkadaşım alone is the normal, natural choice.

Why do we use senin and not just sen?

In Turkish, when a clause is turned into a noun phrase (a subordinate clause, like that you are kind), its subject normally takes the genitive case (often translated as of or ’s in English).

sen = you (subject in a normal sentence)
senin = your / of you (genitive)

In the structure:

senin ne kadar nazik olduğunu
literally: the fact of *your being so kind*

So in these -DIK / -(y)ış / -(mA) type subordinate clauses, you usually need the subject in genitive:
senin nazik olduğunu, onun geldiğini, bizim gittiğimizi, etc.

Can senin be omitted? Is Arkadaşım, ne kadar nazik olduğunu söyledi. also correct?

Yes, you can omit senin if the subject is clear from context:

Arkadaşım, senin ne kadar nazik olduğunu söyledi.
Arkadaşım, ne kadar nazik olduğunu söyledi.

Both are correct. With senin, it’s a little more explicit and formal. Without it, it’s more colloquial, and context must tell you who is being described as kind.

Turkish often drops elements that are obvious from context, especially pronouns and genitive subjects of subordinate clauses.

What exactly does ne kadar mean here? How is it different from çok?

ne kadar literally means how much / how many, and more generally how (to what extent).

ne kadar nazik = how kind (what degree of kindness)
çok nazik = very kind

So:

Arkadaşım, senin ne kadar nazik olduğunu söyledi.
= My friend said how kind you are.

If you said:

Arkadaşım, senin çok nazik olduğunu söyledi.
that would be closer to: My friend said that you are very kind.

ne kadar introduces the degree in an open way (how…), often echoing exclamations like:
Ne kadar naziksin! = How kind you are!

Why is it nazik olduğunu and not naziksin?

Because in Turkish, when you report what someone said without directly quoting their words, you normally use a subordinate clause with a verbal noun, not a finite verb like naziksin.

Direct speech:
Arkadaşım: “Sen ne kadar naziksin.”
(My friend: “How kind you are.”)
Indirect / reported speech:
Arkadaşım, senin ne kadar nazik olduğunu söyledi.
(My friend said how kind you are.)

In indirect speech:

naziksin (you are kind)
→ becomes a nominalized structure: nazik olduğunu
literally: your being kind

So naziksin can’t appear directly inside this structure; it needs to be converted to the …olduğunu form.

What is inside the word olduğunu? How is it built?

olduğunu comes from the verb olmak (to be / to become) and a nominalizing suffix.

A simple breakdown:

ol- = the verb root (to be / to become)
-duğ- = a form of the nominalizer -DIK (changes to -dık / -dik / -duk / -dük / -tuğ / -tüğ / -duğ / -düğ according to vowel harmony and consonant harmony)
-u = 3rd person singular possessive (his/her/its / its being)
-u = accusative case (object marker)

Altogether:

olduğunu ≈ his/her/its being (in the accusative)

In context:

senin ne kadar nazik olduğunu
literally: the fact of your being so kind (and this whole chunk is the thing that was said).

Does the -duğ- part in olduğunu make it past tense?

No, not here. That -duğ- is part of the -DIK nominalizing suffix, not the past-tense suffix -dı / -di / -du / -dü.

In geldi (he/she came), -di is the past tense.
In geldiğini (that he/she came / has come), -diğ- belongs to the -DIK nominalizer.

In olduğunu, the time reference is usually understood from context and from the main verb söyledi; the -DIK nominalization itself doesn’t simply mark “past tense” the way English does. It just turns the action/state into a noun-like structure (a “fact/event of …”).

Here, it expresses something like the fact that you are (were) kind, and the overall time is anchored by söyledi (said).

Why does olduğunu end with -u? What case is that?

The final -u is the accusative case marker.

The entire clause senin ne kadar nazik olduğunu functions as the direct object of söyledi (said). Compare:

Arkadaşım bunu söyledi. = My friend said this.
Arkadaşım, senin ne kadar nazik olduğunu söyledi.
≈ My friend said this fact: that you are so kind.

In the second sentence, instead of bunu, we have a whole clause. To show that this clause is the object, Turkish puts -u (accusative) at the end of the nominalized verb: olduğunu.

Can the word order change? For example, is Senin ne kadar nazik olduğunu arkadaşım söyledi. possible?

Yes, Turkish word order is quite flexible, and the basic meaning stays the same, though emphasis shifts.

Some possible variants:

Arkadaşım, senin ne kadar nazik olduğunu söyledi.
Neutral: My friend said how kind you are.
Senin ne kadar nazik olduğunu arkadaşım söyledi.
Emphasis on arkadaşım: It was my friend who said how kind you are (not someone else).
Senin ne kadar nazik olduğunu, arkadaşım söyledi.
Similar to (2), comma can mark a slight pause/emphasis.

The key is that senin … olduğunu must stay together as a unit; you don’t split senin away from olduğunu by putting other material in the middle (except for things that logically belong inside the clause).

How would this look in direct speech in Turkish?

You can turn the indirect speech into a direct quote like this:

Arkadaşım dedi ki: “Ne kadar naziksin!”
= My friend said: “How kind you are!”

or more simply:

Arkadaşım: “Ne kadar naziksin!” dedi.

The relationship:

Direct: Ne kadar naziksin!
Indirect: senin ne kadar nazik olduğunu (as the object of söyledi / dedi)

Could we use dedi instead of söyledi here? Is there a difference?

Yes, you can very naturally say:

Arkadaşım, senin ne kadar nazik olduğunu dedi. (less common)
More commonly: Arkadaşım, senin ne kadar nazik olduğunu söyledi.

General tendencies:

demek (dedi) is often used:
- with direct quotes: “Ne kadar naziksin!” dedi.
- or with ki-clauses: Arkadaşım dedi ki, sen çok naziksin.
söylemek (söyledi) is more neutral for say / tell, especially with these -DIK subordinate clauses.

Using dedi with a -DIK clause is not wrong, but söyledi is stylistically smoother and more common in this exact pattern.

How would the sentence change if my friend were the one being told, e.g. My friend was told how kind you are?

You would need to make arkadaşım the indirect object (dative case) rather than the subject:

Arkadaşıma, senin ne kadar nazik olduğunu söylediler.
= They told my friend how kind you are.

Breakdown:

arkadaş-ım-a = to my friend (dative, -a)
söylediler = they said / told
senin ne kadar nazik olduğunu = how kind you are (the thing being told)

Now arkadaşıma is the recipient, and the (unspecified) subject is onlar (they), expressed by the -ler ending on söylediler.

How do I say My friends said how kind you are using this pattern?

Just make arkadaşım plural:

Arkadaşlarım, senin ne kadar nazik olduğunu söyledi.
= My friends said how kind you are.

Here, arkadaşlarım (my friends) is technically plural, but many speakers still use söyledi (3rd singular) for groups.
If you want to be very explicit and formal, you can say:

Arkadaşlarım, senin ne kadar nazik olduğunu söylediler.

Both are heard; söylediler clearly shows the subject is plural they.

Is the comma after Arkadaşım necessary?

No, it’s optional here:

Arkadaşım, senin ne kadar nazik olduğunu söyledi.
Arkadaşım senin ne kadar nazik olduğunu söyledi.

Both are correct. The comma can mark a light pause in writing, but it does not change the grammar in this sentence. The main grammatical clues are:

arkadaşım (subject, 3rd person)
söyledi (3rd person singular verb agreeing with it)