principle: Noun A fundamental truth or proposition that serves as the foundation for a system of belief or behavior or for a chain of reasoning. A rule or belief governing one's personal behavior.
Adjective First in order of importance; main. Noun The person with the highest authority or most important position in an organization.
Principal is the correct spelling in this case.
cam2644. Actually, no. The "principal actors", referring to a leading role, is a common phrase. "Principle actor" is not, and does not make sense..
Although, an actor might be "principled" (having ethical principles)-- but that is something different.
This is only partly correct (Your first sentence is correct but this is not the only use of "principal"; your second sentence is incorrect.). See this link for a few examples: http://grammarist.com/spelling/principle-principal/ or this link for more info: http://www.quickanddirtytips.com/education/grammar/principal-versus-principle?page=all
Principle is only a noun and refers to a natural, moral, legal rule or standard. Principal is a noun and adjective with specialized meanings in finance and law but most commonly used to refer to someone in a position of authority or high prominence. (Oxford English Dictionary)
As of May 9, 2018, this still hasn't been corrected, despite dac123's comprehensive and correct comment five years ago. The correct English spelling/term in this case is principal actors (or principal cast/principal cast members in some cases.).
I would really prefer to have Duolingo focusing on corrections like this instead of crowns/color changes and that sort of thing.
¡Órale! Five years ago this complaint was made, and added to from time to time over the last five years. And no one has corrected this?
Well, I came here to say "principle" is wrong but so many have already done it that it's unnecessary. the duolingo people really need to change this one. It is embarrassing.