long

Compared meaning: lasting for a long time (adjective).

比较义项：持续时间很长的（形容词）。

• lengthy

  Formal word implying something is tediously or excessively long; typically describes meetings, speeches, explanations, or documents.

  较正式的词，暗示某事冗长乏味，多用于会议，演讲，解释或文件。

  The professor gave a lengthy explanation before finally answering the question.

  教授先给出了一段冗长的解释，然后才终于回答了问题。

• long 本词

  The everyday, neutral word for a long duration; combines freely with almost any noun and fits the common phrase 'for a long time'.

  最普通中性的说法，几乎可以搭配任何名词，也常用于“for a long time”这样的短语。

  We've known each other for a long time, since primary school.

  我们从小学起就认识了，已经很长时间了。

• lifelong

  Means lasting for someone's entire life; used only for things tied to a whole lifetime, such as friendships, dreams, or early habits.

  指贯穿一个人的一生，只用于伴随终生的事物，如友谊，梦想或从小养成的习惯。

  They became lifelong friends after meeting in college.

  他们在大学相识后，成为了终身好友。

• endless

  Emphasizes that something feels like it has no end, often used with exaggeration to express boredom, frustration, or impatience.

  强调某事仿佛没有尽头，常带夸张语气，表达厌烦，沮丧或不耐烦的情绪。

  Waiting in that line felt endless, so we finally decided to leave.

  排那个队感觉没完没了，我们最后决定离开了。

Lengthy, long, lifelong, and endless all mean lasting a long time. Lengthy means tediously long, like a speech. Long is neutral. Lifelong means lasting a whole life, for friendships or dreams. Endless feels like it never ends. Not covered: long can mean far away.

这四个词都可以表示“持续时间很长”，但语气和用法不同：lengthy强调冗长，常显得乏味，多用于正式场合；long是最普通中性的说法；lifelong特指贯穿一生，只能用于友谊，梦想等伴随终生的事物；endless则带夸张语气，强调“感觉没有尽头”，常表达不耐烦。这里不比较long表示“距离长”的义项（如a long road）。

Compared meaning: lasting for a very long time (adjective).

比较义项：持续时间很长、久远（形容词）。

• eternal

  Strongest of the four: suggests something lasts forever with no end, often used in emotional, romantic, or religious contexts rather than everyday facts.

  语气最强：表示某事物永远持续、没有尽头，常用于爱情、生命或宗教等情感色彩较浓的语境，而非普通的日常事实。

  Many religions promise eternal life to those who follow their teachings.

  许多宗教向那些遵循其教义的人承诺永恒的生命。

• long 本词

  The most general, neutral word for duration, used for almost anything that continues for a great amount of time, without eternal's sense of forever.

  语气最中性、最通用，几乎可以形容任何持续很久的事物，但不像eternal那样带有“永远”的意味。

  It was a long, tiring day at the office.

  那是在办公室度过的漫长而疲惫的一天。

• extended

  Implies a period that was deliberately made longer than originally planned or than usual, often used for stays, deadlines, or vacations.

  表示某段时间被特意延长，超过了原定计划或通常的时长，常用于假期、逗留或截止日期等场合。

  We asked our professor for an extended deadline on the final project.

  我们请求教授延长这份期末项目的截止日期。

• lifelong

  Specifically spans a person's entire life, used mainly for relationships, habits, or pursuits rather than short-term or abstract things.

  特指贯穿一个人的一生，主要用于人际关系、习惯或追求，而不是短期或抽象的事物。

  They have been lifelong friends since they met in kindergarten.

  自从在幼儿园相识以来，他们就是终身的朋友。

Eternal, long, extended, and lifelong all mean lasting long. Eternal is strongest, meaning no end. Long is the most general, neutral duration word. Extended means longer than planned. Lifelong spans a whole life, for relationships or habits. Not compared: long as distance, extended as stretched.

这几个词都可以表示持续时间很长，但语气强弱和使用场合不同：eternal语气最强，表示“永远、没有尽头”，常用于爱情、生命或宗教等情感色彩浓厚的语境；long是最普通、中性的说法，几乎可以形容任何持续较久的事物；extended强调“比原计划或通常时长更长”，常用于假期、逗留、期限等被特意延长的情况；lifelong特指“贯穿一个人的一生”，多用于友谊、习惯或追求。另外，long本身还有“（距离）长的”这一空间意义，extended也有“伸出的”这一物理意义（如伸出的手臂），这两种意义这里都不作比较。