Nan means “what” and is used before such sounds as D(ex. Koreha NAN desuka?), N(ex. Kore ha NAN no hon desuka?) and T(NANto!!＝showing that you are surprised). Nan also means “How many” if it is used with a counter. (ex. Nan nin =How many people?, Nihongo ha nannen benkyou shiteimasuka?= How many years have you been studied Japanese?) Nani means “what” as well. Ex. Kore ha Nani iro desuka? =what color is this? Anata ha nani wo shiteimasuka?= what are you doing? ※There are some exceptions. For example, 何回生？（Nankaisei）＝What is your grade at the university? Kyou ha NANyoubi desuka?=what day is it today? -In such cases, it may be easier to pronounce NAN than Nani. Good luck! Ganbattekudasai ^^*