Principal and Principle
This is one of the most common mistakes between two words in American English. These two words actually share a very distant root in the Latin word "princeps"; however, they diverged in meaning a very long time ago. In fact, even though these two words may sound similar, they are very different in their meaning. The confusion in the use of these two words is very prevalent and it is found almost daily in news media, University web pages, job postings, etc.
A person that holds the highest rank or responsibility
“The principal contributor to this blog is Emma Perez-Costas”
“The principal investigator has the highest responsibility in the planning, execution, and reporting of this science project”
Note that, It can be also used in a plural situation in which there are several people that hold equal responsibility, but the word principal does not have a plural form when used with this meaning.
"The principal contributors of this blog are John Doe and Jenny Nemo"
"The principal investigators carried out the planning of all the experiments"
The capital invested or owed before the interest and fees are taken into account
The principal owed in a house mortgage is the amount borrowed from the bank or lender.
“The principal of my house mortgage is 150,000 dollars”
The post of "principal" in an educational institution such as a primary school
"The principal of my school is leaving at the end of the academic year"
Principle has a very specific meaning and it is only used as a noun, and can be used in the singular (principle) or plural (principles) form.
A principle can be a rule, a law, something considered a basic truth, a belief or group of beliefs...etc.
"I stand by my principles"
"My principles prevent me from participating in this business"
"The principle that rules this scientific hypothesis is..."
Finally, here there is an example of using principal and principle in the same sentence
" The principal reason why I left the Board of Trustees was because the resolutions taken were against my principles"