【なるほど(ね)・naruhodo(ne)】I see/makes sense/now I know
なるほど(ね)・naruhodo(ne) = I see/makes sense/now I know
You can use this to say:
Oh I see.
That makes sense.
Now I know.
"Naruhodo" originally means "as much as possible". When something is "as correct as possible", or "as true as possible", you express your understanding by saying "Naruhodo".
"Naruhodo" shows you are amazed or impressed by it, but in real life, this word does not mean that much, and it's used casually as "I see."
When your question is answered to your satisfaction, you would say "Naruhodo". However, this may sound a little arrogant, so better not use it to your superiors. (Instead, you can just say "Wakarimasita (I understand)".
Between friends, you can say "Naruhodo ne". "ne" makes the sentence casual and friendly, so unless you are sure, refrain from using it toward superiors.
EXAMPLES
I see.
Grace
この ほうほう は、むかし から せかいじゅう で つかわれて いるんです。
(kono hōhō wa, mukasi kara sekaijū de tukawarete irundesu.)
(This method has been used all around the world for a long time.)
なるほど。それはしりませんでした。
(Naruhodo. Sore wa sirimasen desita.)
Keiko
(I see, I wasn't aware/I didn't know that. )
It makes sense
Grace
そのもんだいは、このすうしきをつかって、とくんですよ。
(Sono mondai wa, kono sūsiki o tukatte, tokundesuyo.)
(You solve the problem using this formula.)
なるほどね!あなたはてんさいですね!
(naruhodo ne! anata wa tensai desune!)
Keiko
(Now I know! You are a genius!)
A tip
"Naruhodo(ne)" is used more often by adult male. Even though used casually without deep meaning, the word still sounds rather academic or rigid. For you to say "Naruhodo(ne)", you must have achieved some knowledge or meaningful understanding and are impressed.