ABOUT:
One of the most useful questions words is なに or なん. Both are pronunciations of the kanji 何 and both mean "what". So, how do you know which pronunciation to use?
Quick and Dirty なん: Pronounce 何 as なん if it comes before a word beginning with T, D, or N. For the most part everything else is pronounced なに. If this sounds complicated, just learn examples and you'll eventually get it.
For Example:
■ 何ですか？ What is it?
[It is なん because it is before a "D" sound.]
■ 英語では、何というのですか？ How do you say that in English?
[It is なん because it is before a "T" sound.]
This is a good rule, but here are a few other "rules" that might be more concrete.
なに
Pronounce 何 as なに if it is by itself or before a particle. Also pronounce it as なに if before a noun (except when asking what day of the week).
■ 何？！ What?! [By itself.]
■ 何が好きですか。 What do you like? [Before a particle.]
■ 何色が好きですか。 What color do you like? [Before a noun.]
■ 何もほしくないです。 (I) don't want anything. [Before a particle.]
なん
なん: Pronounce 何 as なん before counters, when asking the day of the week (the exception above).
■ 何回 how many times [Before a counter.]
■ 何枚 how many pages (of paper) [Before a counter.]
■ 何曜日 what day of the week [The exception before a noun.]
Other Forms:
■ 何も nothing; not any [Takes a negative verb.]
-> 何も分かりません。 I don't understand anything.
■ 何か something
-> 何か飲みますか？ Would you like something to drink?
■ 何でも anything; whatever; any
-> 何でもいいです。 Anything is fine.
■ 何で Why? What for?
-> 何で来なかったの？Why didn't you come?
