"WEATHER" and "WHETHER"

"Weather" refers to the atmospheric conditions. "Whether" is used to introduce alternatives or choices. Ex.: The weather is perfect for a picnic. I'm not sure whether to wear a jacket or not.

"PRINCIPAL" and "PRINCIPLE"

"Principal" refers to a person in a position of authority or the primary part of something. "Principle" means a fundamental rule or belief. Ex.: The school principal addressed the students. The principal reason for our success is teamwork. Honesty is an important principle to uphold.