On moral grounds alone, writers ought to properly use principle and principal.
Principle means “a basic truth or belief”: Liberty is among the principles the United States was founded upon. It is always a noun.
Principal refers to “something of primary importance.” It could be the guy who heads the school (The principal gave the naughty student a detention.). It might be money in a loan (The principal on his car came to several thousand dollars.). It could describe why something was done (The principal reason he retired from his job was frustration with his boss.). Note that it can be both a noun and an adjective.
So don’t ever get this wrong again, dear writer – or we’re sending you to the principal’s office.
My name is Rob Bignell. I’m an affordable, professional editor who runs Inventing Reality Editing Service, which meets the manuscript needs of writers both new and published. I also offer a variety of self-publishing services. During the past decade, I’ve helped more than 300 novelists and nonfiction authors obtain their publishing dreams at reasonable prices. I’m also the author of the 7 Minutes a Day… writing guidebooks, four nonfiction hiking guidebook series, and the literary novel Windmill. Several of my short stories in the literary and science fiction genres also have been published.