何

何 generally means “what.” It can also mean “which” and “how many” when combined with other word.
何 can be pronounced as  なに (nani)  and なん (nan), depending what is connected to it.

何 (なに)

なに is usually seen as a one complete word, therefore, it is usually used with particles like a noun.
Examples,

• Q: それは何？(kore wa nani) = What is this?
  A: これは本 。(kore wa hon) = That is a book.
• Q: 何にしますか？(nani ni shimasu ka) = Which one you choose
  A: それにします。(sore ni shimasu ) = I’ll choose that one.
• Q: 何で行くか？(nani de iku ka) = How will you do it?
  A: 電車で行く。(densha de iku) = I will go by train.
• Q: 何があったの？(nani ga atta no) = What happened?
  A: 事故があった。 (jiko ga atta) = An accident happened?
• Q: 何をしますか？ (nani wo shimasu ka) = What are you going to do?
  A: 祭りに行く。 (o matsuri ni iku) = Go to a festival.

なに [noun] = Which …, What …

• Q: 何色が好きですか？ (nani iro ga suki) = 「What color you like?」 OR 「Which color you like? (limited options)」
  A: 青色です。 (aoiro desu) = 「Blue」 OR 「The blue one.」 
• Q: あなたの好きな色は何色？ (nani iro ga suki) = What is your favorite color?
  A: 青色です。 (aoiro desu) = Blue
• Q: 何空港に着きますか？ (nani kuukou ni tsukimasu ka) = At which airport will you arrive?
  A: 関西空港に着きます。 (Kansai-kuukou ni tsukimasu) = I will arrive at Kansai Airport.
• 何者だ! (nani mono da!) = Who the hell are you?
• 何こと？(nani koto) = What it is about? / What happened?

何 (なん)

なん is usually seen before particles の (no), と (to) and だ/です.
It is also used when the connected word begins with a syllable t, n, or d.
When なん is connected by a counter, it means “how many.”

[V1] の [N2] is to describe characteristic of N2 by N1.
なん の [N2] is used to ask for the description.

• Q: 何の本ですか？ (iro ga suki) = What kind of book? / What the book is about?
  A: 歴史の本 (reikishi no hon) = History Book
• 何のこと？= What it is about?  / What happened?

[Quotation] と: と in this case is for a quotation, not to indicate accompanying person.

• Q: 何と思う？(nan to omou) = What do you think?
  A: いいと思う。(ii to omou) = I think it’s good.
• Q: 英語でなんと言いますか？(eigo de nan to iimasu ka) = How to call this in English?
  A: 「Apple」と言います。(Apple to iimasu) = It’s called “Apple.”

なん [counter] = How many …? , What …?

• Q: 何人 (nan-nin) = how many person? / how many people?
• Q: 何回 = how many time?
• Q: 何本 = how many? (long, skinny things)
• Q: 何時 = what time?
• Q: 何曜日 = what day of a week?

Connecting word begins with a syllable t, n, or d.

• なんだ、それは？(nanda sorewa) = What’s that?
• パソコンってなんなの？(Pasokon-te nan na no) = What is “Pa so kon”? ** Personal Computer

Learn Vocabulary

https://www.youtube.com/playlist?list=PLB3Mq8bM3diGTsTlV2LrCtxF12bHIwgA8