The homophones principal and principle are often confused with each other. Although they derive from the same root, there is no current overlap in meaning. Even when the difference has been taught in school, it is easily forgotten, so a quick recap and some mnemonics might help keep their meanings mutually distinct.
Principal is usually an adjective that means main, chief, primary, most important, most influential, or highest ranking:
Job satisfaction was the principal reason for her decision.
Cocoa is the principal ingredient.
As a noun its chief meaning is principal official, main participant or person of primary rank:
All the principals attended the company’s AGM.
After eight years at the school he became its principal.
To remember this you could think of the a common to principal, primary, and main, or the -al at the end of both principal and official.
Principal (n) also has various financial and legal definitions, e.g. the person directly responsible for a crime; the person represented by an agent or proxy; the main body of capital, not counting interest or revenue. You are unlikely to use the word in these technical senses unless you already do so frequently, in which case you are more likely to know its spelling.
Principle is only ever a noun, and means a rule, doctrine, standard of conduct, general scientific or natural law, or basis of a system of thought or action.
He replaced the damaged item as a matter of principle.
Her lifestyle was founded on the principles of honesty, simplicity, and kindness.
Corruption undermines the principles of democracy.
Self-similarity is a fundamental principle of fractal geometry.
Principle is commonly used in the phrases in principle, which means in theory, and on principle, which means because of one’s beliefs or ethical code:
I agree with the idea in principle, but we will need to discuss it in detail.
We recycle electronic waste on principle, not because it is more convenient.
It also forms the adjective principled, which means based on, or characterised by, principles, i.e. acting in a decent and moral way:
They were regarded as a principled organisation, one that made decisions based on enlightened self-interest rather than raw profit.
Again, a mnemonic can help distinguish the words. Think of the -le in both principle and rule. Or, since principal is usually an adjective and principle is always a noun, think of the principal principle, with a coming first as it does in the alphabet. For a more vivid aid to memory, you could picture a head teacher named Al, Al the Principal, whose principal reason for marrying Elly (“l e”) is that she is a woman of principle.
Just try not to mix them up…