Is なの the casual form of んです or is んだ?

Question

For example:

• なぜなの？
• なぜなんですか
• なぜなんだ？

Do those sentences have the same meaning?

I also learnt that there is an even more formal way to express the んです and is with のです and のだ.

And I also hear that なの is a female expression, is that true? Does a male version exist?

Answer 1

のです is a constructuon that:

• asks for or gives an explaination or information (or additional infos)

今日は仕事に行けません。病気なんです。

• when you want to be sure that what you imagined or judged from a context is true or not

(Someone entering the office with an umbrella) 雨が降っているんですか。

• when you express your interpretation or you understand something that wasn’t clear before, the sentence ends with のだ/んだ

きっと結婚しているんだ！

ああ、ここで事故があったんだ

A question asked with the のです contruction could sound persistent and very direct so you shouldn’t use it with people you’re not close or that you don’t know very well (its usage is difficult for many foreigners). んです or んだ are used when speaking and not as a writing form.

な used before の may be there because of the word preceding it:

you don’t say なぜの？ but なぜなの？

It’s 難しいの？ and not 難しいなの？

You say 風邪なの？ to ask “is it a cold?”. While 風邪の？ could exist off course but it should be contestialized is a discussion and you’re not asking if it’s a cold but something related to the cold we’re talking about.

Never heard about this structure to be used mainly by girls.

Answer 2

Yes, they are synonymous. In fiction, frequently use of なの is a feature that's unique to girly characters, while in reality men use it too.