Differencen between NAN and NANI

There is no reason for why NANI is used over NAN. This is just the way it is. Some words use NAN and some use NANI. You normally can not choose which one to use, but must use the official version. Here are some example words:

NANI (what) which actualy can be NAN whenever a particle isn't used. For example NAN DESU KA (What is it?) does not have a particle, but a sentence like NANI O KAIMASU KA (What will you buy?) and NANI GA SUKI DESU KA (What do you like?) has a particle so NANI is used. The exception to this unnoficial rule is when the particle NO is used. NANI NO is never said, but instead NAN NO is used as in SORE WA NAN NO BIIRU DESU KA (What beer is that?).

NANI-IRO (what color) is always NANI and never NAN. You need to remember that in Japanese "what color" is not 2 words but is actually one word NANI-IRO. I used the hyphen just to make it easier to read.

NANSAI (how old). Even though "how old" is two words in English in Japanese it is just one word NANSAI.

These are just a few examples. There really is nothing to think about here. You just need to memorize the question words as they are. Try not to focus on the fact that the word WHAT is in them because they are independent words that coicidentally have NAN or NANI as a portion of the word. It is important not to over analyze them. Keep them as independent from what I explained about NANI above since they don't have anything to do with this and DO NOT change. Only NANI by itself changes and not when it is part of the word.