you can use them as making agreeable responses. I can't explain well, but I'll try "そうなんだ"　is often used when people understand about what someone talking. for example, imagine that teacher tell you new things, you can say "そうなんだ" or "そうなのか" when you completely understand it. next, "そうそう" is often used in usual conversation. It is used when you agree someone's opinion. for example, when your friend say that our English teacher always give us too much homework,you can say "そうそう" if you agree your friend's opinion. Finally, "なるほどね" mean "I see" in English. it is also used when you understand something. for example, imagine that you're waiting in front of restaurant for your friend. but your friend is late. so you hear the reason why he is late to him. then he said "Because I lost my wallet on my way to the restaurant" then you can say ”なるほどね。それで、財布は見つかったの？”(I see, anyway, did you find your wallet?) I don't know if you understand. I'm sorry, my explanation is very bad.

そうなんだ　此意思是“是这样的” そうそう 此意思是“是的，是的” なるほどね 此意思是“原来如此”

「sou-nanda」 "sounanda" is commonly used when agreed with depression. The conversation on the other, "Yeah," often used to "sou-sou"is mostly "yeah" "Its alright" "Oh" In addition, "That reminds me..." is also used different words. for exanple like 「sousou(Oh), what happened about that?」 "Naruhodone" means "I see" , "I got it" I veeeeeeery often use "sou-nanda"