PRINCIPLE - [noun] a rule or idea that explains how something works
Examples of use
Every student of Marketing and Management must be familiar with principles of reasonable investment.
All of us should know some general principles of law. It could reduce negative consequences of our decisions.
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English speaking practise
Remember that if you don't use that word in English conversations or writing you'll forget it in a short time. So the best thing you can do to learn this word is just trying to use it in speaking or writing. Remember also that practise is the key to fluent english speaking.
You can always find a partner, who would also like to improve his/her