Arkadaşım, senin ne kadar nazik olduğunu söyledi.

to be

Breakdown of Arkadaşım, senin ne kadar nazik olduğunu söyledi.

• benim: my
• arkadaş: the friend
• sen: you

Questions & Answers about Arkadaşım, senin ne kadar nazik olduğunu söyledi.

In this sentence, is arkadaşım the subject (my friend) or am I calling out to someone like my friend, …?

Here arkadaşım is the grammatical subject: my friend.

• 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 ?