learn the rule
principle / principal
principle is a noun. It means a fundamental rule, belief, or law that guides behaviour or thinking.
- It's a matter of principle.
- The principle of supply and demand.
- I refuse on principle.
principal has two main uses.
As a noun, it usually refers to the head of a school, the most important person in a group, or the original amount of money in a loan or investment.
- The school principal wants to see you.
- She is the principal of the firm.
- The loan repayment covers both principal and interest.
As an adjective, it means main or most important.
- The principal reason for studying is to learn.
- Potatoes are the principal crop of this farm.
- The principal reason for this site is people like you.
- Your principal problem is your spelling.
The two are not interchangeable.
A way to remember: the principal at school is your pal (or at least pretends to be). A principle is the fundamental rule.
Getting it wrong makes you look stupid. And ugly. The principal problem is the principle that you can't tell them apart.