What additional meaning that なんだ add to the sentence


A. 彼は大学生ですよ  
B. 彼は大学生なんだよ  
What difference that なんだ makes in this sentence?

For A, I understand it well as "He is a college student!"  
For B, eventhough i have read much article written about なんだ... I still cannot
understand it.

I notice that there are な+んだ or 何だ.  
I feel that it was the な のだ, but i still wasn't sure as to how it changes the
meaning of sentence.




In this case, なんだ is used for emphasis. For example, you can use it if you are
answering someone's question and you are confident in your answer. Let's say
for example that someone asked you if so-and-so is a student, and the
questioner isn't certain (doubting) that they are a student, you can say はい,
せいとなんだ. (Yes, he/she **is in fact** a student)

