fromEnglish Grammar Today
Principalas an adjective means ‘most important’:
Theprincipalreason for the failure to take action was poor communication between government departments.(the most important reason)
We can useprincipalas a noun to mean the head of a school or college (especially in American English):
The collegeprincipalmade a speech congratulating all the students who were graduating in that year.
Principleis a noun. It means ‘a rule or theory which explains how something is or works’ or ‘a moral rule or guideline’:
The scientificprinciplesbehind even the most complicated computer are relatively simple.
He seems to have noprinciplesat all, and is only interested in money.(He has no moral rules or guidelines.)