A Part-of-Speech Approach
Instead, let’s take a simple parts-of-speech approach to understanding the differences. The word principle will always appear as a noun form, never as an adjective. It is thus incorrect to say:
This was the principle (meaning main) reason supporting the company’s decision.
A Trick to Help You Remember
Also, the word principle essentially means just one thing (or variations of this one thing): a rule or tenet or precept or policy. The key word to remember is rule. If tricks help, both words end in “‑le.” Here are some examples:
Some people have high moral principles (rules). An inventor’s research must adhere to scientific principles (rules). The court followed the sound judicial principle (rule) of interpreting a statute in light of legislative intent.
The Word Principled
The word principle will operate as an adjective only in its noun past-participial state. In the English language, you can take a good old noun and turn it into what amounts to a past participle. Thus: the wooded lot, the two-faced politician, the hooded robber. (Notice how these verb-like words derive from the nouns wood, face, and hood.)
In your writing, you might have the occasion to describe a decision of your boss as a principled decision.
But other than this -ed version, the word principle acts not as an adjective but only as a noun. It means rule. Remember:
The word rule ends in -le and so does the word principle.