how to use そうなの and そうなんだ?
I want to know when to use そうなの and when to use そうなんだ.
does it have to do with past tense or present tense?

Question

how to use そうなの and そうなんだ?
I want to know when to use そうなの and when to use そうなんだ.
does it have to do with past tense or present tense?

Answer

そうなの？　is used when you don't know the fact.
So it's like "Oh really??''
Or under certain situation, that means the same as そうなんだ.
It depends that you ppu"?'' or not. 
And そうなんだ is used when you know the fact.
So it's like "Oh I see.''
I hope this would help you!!