necessary

Compared meaning: needed, required for something (adjective).

比较义项：必要的，为达成某事或使某情况发生所需的（形容词）。

• necessary 本词

  The neutral, general-purpose word for something needed to achieve a goal or make a situation happen; works in almost any context, formal or casual.

  语气中性、最通用的词，指为达成目标或使某种情况发生所需要的东西，几乎适用于任何正式或非正式场合。

  It's necessary to book a table in advance on weekends.

  周末最好提前预订餐桌，这是必要的。

• essential

  Stronger than necessary; stresses that something is absolutely indispensable, so central that nothing can work or succeed without it.

  语气比 necessary 更强，强调某事物是绝对不可或缺的，核心到没有它就无法运作或成功。

  Regular exercise is essential if you want to stay healthy long-term.

  如果想长期保持健康，规律运动是必不可少的。

• requisite

  A formal, somewhat literary word, most often placed before a noun (the requisite documents/skills) rather than after 'is'; common in official contexts.

  书面语气较为正式，略带文学色彩，通常放在名词前使用（如 the requisite documents/skills），而不是放在系动词之后；常见于官方或公文语境。

  She finally gathered the requisite paperwork to renew her passport.

  她终于备齐了续办护照所需的必要文件。

• obligatory

  Means required by a law, rule or convention rather than by practical need; you must do it because you are told to, with no choice.

  表示因法律、规则或惯例而被要求，而不是因实际需要；必须去做是因为被规定如此，没有选择余地。

  Wearing a helmet is obligatory for all construction workers on site.

  工地上所有建筑工人都必须戴安全帽，这是强制规定。

Necessary, essential, requisite, and obligatory all mean needed or required. Necessary is the general word. Essential is stronger, meaning something is truly indispensable. Requisite is formal and literary, usually placed before a noun. Obligatory means required by a rule or law, not by real need.

这几个词都表示“必需、不可或缺”，但语气强弱和使用场合不同：necessary 最通用中性，essential 语气更强调绝对不可或缺，requisite 书面正式，obligatory 则强调因规则或法律而必须做，不是因实际需要。