“nǐhǎo” is basically
the first phrase that you learn in Chinese class. People tend not to use it so
much with people who they’re at all familiar with. It is kind of awkwardly
formal.
“nínhǎo” is the respectful form of “nǐhǎo” – it’s used with
people whom you want to express respect towards (a teacher, perhaps). “nínhǎo”
is actually used and appropriate in such situations.
