Which word in the sentence is a noun? a principal speaks.

the answer principal 

A noun is a word that functions as the name of some specific thing or set of things, such as living creatures, objects, places, actions, qualities, states of existence, or ideas.

Based on my sentence above your noun would be Speak its the action the princible takes.

Example :
when you have one it's family,but when you have two it's families.