In Turkish there is a root word and suffixes are added to build more complex ones. In this case iyi, the root, is used as a verb and the subject of the sentence is added as a suffix. As such in oral conversation the subject pronouns are often not needed (kind of like Spanish)
Ben iyiyim - I am fine
Sen iyisin - You are fine
O iyi/iyidir - He/She/It is fine
Biz iyiyiz - We are fine
Siz iyisiniz - You are fine
Onlar iyiler/iyidirler - They are fine
I hope this helps.
In English, we might say "it's fine" where "it" is kind of semantically empty to say "the situation is fine" or, more figuratively, "don't worry about it."
...Can you say "O iyi" in the same way to mean "it's fine" like you would in English? Or would the "O" be emphatic (e.g., "HE/SHE/IT is fine"), so you could just say "iyi"?
Edit: Some googling makes me think that the equivalent of "it's fine" is "gerek yok."
Is there no difference between "ben iyiyim" and" iyiyim" ? because sometimes I wrote BEN IYIYIM in translation of I AM FINE but then it becomes wrong.
They have the same meaning "I am fine" & when you use the personal pronoun "ben" you emphasize the individual "yourself."
Can you say "what" becomes wrong? Please post what you have written here for me to see?
Turkish is suffix based language and the root gets a number of suffixes attached depending on person, question, verb tense, etc. However, often suffixes has a number of versions to have vowel harmony ("sesli uyumu" in Turkish). The following link should help you:
Musun is not a word itself. mu is the question suffix and is always written apart sun is another suffix. But consider musun with the verb that comes before it. For example:
Okuyor musun? = Are you reading?
Oku = To read
yor = present continuous tense suffix in Turkish.
mu = question suffix
sun = suffix for the 2nd person singular (you)