Both principal and principle come from the same root word in Latin, primus, or prime. The two words are linked, which accounts for their distinct but similar meanings.
principal和principle都来自于同一拉丁语词根，primus或prime。这两个单词虽然不同但相似的含义使得这它们常被联系起来。

Principal

Principal has two meanings, one as an adjective and the other as a noun. However, they have the same pattern: they mean something or someone that is a leader, the first, and/or the most important.
Principal有两个意思，一个是作为形容词，另一个是作为名词。这两个单词尽管词性不同，却有相同含义：皆为某事或某人是主导/领导，第一个或最重要的。

Noun: the head, chief, or leader; the leader of a school; the initial sum of an investment
名词词性：首脑、首席或领导；学校的领导；投资的初始金额

Adjective: the most important, the chief; referring to the initial sum of an investment
形容词词性：最重要的，主要的；指投资的初始金额。

Principle

Principle is always a noun, unless the word is in a different form.
Principe除变形外只为名词。

Noun: A truth or idea that serves as the foundation for something, such as a belief, behavior, or moral code; an accepted rule for conduct
名词词性：作为某事基础的真理或想法依据，如信仰、行为或道德准则；公认的行为准则

Principled is the adjective form of principle. It simply describes something (a person, moral code, etc.) that is founded on principles.
Principled是principle的形容词形式。只用来描述基于原则的事物（如，人，道德准则等）。