Welcome to some more of those tricky words.
principal / principle
Principal is a noun that means a person who holds a high position or plays an important role: “The school principal has 20 years of teaching experience. The principals in the negotiations will meet tomorrow at 10 o'clock.” It also means a sum of money on which interest accrues: “The depositors were guaranteed they would not lose their principal.” Principal is also an adjective that means chief or leading: “The necessity of moving to another city was the principal reason I turned down the job offer.”
Principle is a noun that means a rule or standard: “They refused to compromise their principles.”
soap / detergent
Soap, a noun, is a cleansing agent that comes from natural fats and oils.
Detergent, a noun, is a cleansing agent that is usually made from synthetic, or man made, products.
stationary / stationery
Stationary is an adjective that means fixed or unmoving: “They maneuvered around the stationary barrier in the road.”
Stationery is a noun that means writing materials: “We printed the letters on company stationery.”
Clancy's comment: Mm ... Sound familiar?