Principal : The word can be used as both an adjective and a noun. The principal thing or person in a group is the most important one. P rinciple : The word is only used as a noun. A principle is a basic rule that explains or controls how something works or a guiding rule for behavior. principal principle 1) The _________ aim of the policy is to bring peace to the area. 2) His _________ interest in life was to become the most outstanding genetic engineer (遗传工程专家) in Europe. 3) A bicycle or a motorcycle is built on the same ________, though the force that moves them is different. 4) He never allowed himself to be bullied (恐吓) into doing anything that went against his _________. 5) Complaints from the students began arriving at the ________ ‘s office.