fundamental

Compared meaning: main or most important (adjective).

比较义项：主要的，最重要的（形容词）。

• principal

  Principal is used before nouns to name the chief one among several within an institution or hierarchy (principal shareholder, principal office); it has a formal, almost official tone.

  principal 常放在名词前，用来指出机构或等级中最主要的那一个（如 principal shareholder「主要股东」、principal office「总部」），语气正式、带有官方色彩。

  The principal shareholder of the company owns forty percent of its shares.

  这家公司的主要股东持有百分之四十的股份。

• primary

  Primary highlights the one that comes first in rank or order among several goals, sources, or concerns, and is the standard word in phrases like primary goal, primary source, and primary concern.

  primary 强调在多个目标、来源或考虑因素中排在首位，是 primary goal（首要目标）、primary source（主要来源）、primary concern（首要关切）等固定搭配的常用词。

  The senator's primary goal this year is to lower taxes for working families.

  这位参议员今年的首要目标是为工薪家庭减税。

• important

  Important is the everyday, general word for having great value or effect; it works freely as a standalone claim (it is important that...) without ranking one thing above others.

  important 是日常最通用的词，表示具有重大价值或影响，可以独立作出判断（如 it is important that……），不必暗示要在几者中排出主次。

  It is important to drink plenty of water every single day.

  每天喝足够的水是很重要的。

• fundamental 本词

  Fundamental describes something forming the essential base that everything else depends on, closer to 'foundational' than merely 'important', and pairs with principle, right, or part.

  fundamental 表示构成事物赖以存在的基础部分，含义更接近「基础性的」而不仅仅是「重要的」，常与 principle、right、part 等词搭配。

  Trust is a fundamental part of any strong friendship.

  信任是任何牢固友谊中不可或缺的基础部分。

Principal, primary, important, and fundamental all mean main or most important, but differ in strength and formality. Principal names the chief one in a group. Primary marks what comes first. Important is the general word. Fundamental means the essential base.

这些词在「主要的、最重要的」这一意义上相近，但语气强弱、正式程度和搭配习惯不同。这里不比较 principal 作名词表示「校长」的用法，也不比较 primary 表示「最初级的」（如 primary school「小学」）的用法。