only

Compared meaning: only, merely, nothing more than this (adverb).

比较义项：仅仅，只是（副词）。

• but

  In this sense but is formal and literary, meaning 'only' or 'merely'; it appears mostly in fixed phrases and old-fashioned writing, rarely in everyday speech.

  在这个意义上，but 十分正式、书面化，相当于“仅仅”“只是”，多见于固定搭配和老式书面语中，日常口语里很少这样用。

  He is but a child, and cannot be blamed for this.

  他不过是个孩子，不能因此责怪他。

• just

  Just is the most common, neutral, everyday way to say 'only'; it is casual and fits ordinary conversation and writing of any register.

  just 是表达“仅仅”最常见、最中性的说法，语气随意，适用于任何场合的日常口语和书面语。

  I just need five more minutes to finish this email.

  我只需要再花五分钟就能写完这封邮件。

• simply

  Simply adds emphasis, presenting the limitation in a plain, direct way, and can even shade into 'completely'; here it stresses how plainly true the fact is.

  simply 带有强调意味，把这种“仅仅”表达得更加干脆、直接，有时还可以带有“完全”的语气；这里强调事实的单纯明显。

  She simply refuses to apologize, no matter what anyone says.

  不管别人说什么，她就是不肯道歉。

• only 本词

  Only is the most basic, general, and frequent word for this meaning, used freely in both speech and writing, with flexible position in the sentence.

  only 是表达这个意义最基本、最通用、最常见的词，口语和书面语中都自由使用，在句中的位置也很灵活。

  I only wanted a small slice of cake, not the whole thing.

  我只是想要一小块蛋糕，不是整个蛋糕。

But, just, only, and simply can all mean 'only', but usage differs. But is formal and literary here, rare in speech. Just and only are the most common, natural in speech and writing. Simply is stronger and more direct. But's meaning 'however' is not compared.

这几个词在“仅仅、只是”这一意义上相近，但语气和使用场合不同：but 在此义项下非常正式、书面化，现代口语中很少见；just 和 only 最常用、最口语化，可自由用于书面语和口语；simply 语气更强调、更加直接。这里不比较 but 用作转折连词（意为“但是”）的用法。

Compared meaning: being the only one of its kind (adjective).

比较义项：形容“只有一个”或“独自存在”的状态（形容词）。

• lone

  Lone describes someone or something standing apart, alone in a scene; it is mainly literary, used before a noun, and stresses solitariness rather than being the only one of a kind.

  lone用来描写某人或某物在场景中显得孤零零、独自一个，通常放在名词前，语气偏文学化，强调的是孤单感，而不是“同类中唯一的一个”。

  A lone wolf howled somewhere in the dark forest.

  一只孤狼在黑暗的森林中某处嚎叫。

• only 本词

  Only means no other person or thing fits that role here; it usually follows 'the' and stresses that all others are excluded, not that something stands physically apart.

  only表示在这个场合里没有其他人或物符合这个角色，通常与“the”连用，强调把其他一切都排除在外，而不是强调身体上的孤立。

  She was the only witness who saw the accident happen.

  她是唯一目睹事故发生的目击者。

• unique

  Unique stresses that nothing else is exactly like it; it describes a special or distinctive quality, not physical solitude or exclusivity among a group.

  unique强调没有任何其他事物与它完全相同，描述的是一种特别或与众不同的特质，而不是身体上的孤单或在同类中的排他性。

  This restaurant's fusion menu gives it a truly unique dining experience.

  这家餐厅的融合菜单让它拥有真正独一无二的用餐体验。

• single

  Single simply counts one item as opposed to more than one; it highlights quantity, not exclusivity or distinctiveness, and pairs naturally with countable nouns.

  single只是单纯地表示数量上“只有一个”而不是更多，强调的是数量，而不是排他性或独特性，常与可数名词搭配。

  The whole trip was cancelled because of a single mistake.

  整趟旅程因为一个单一的失误而被取消。

Lone, only, unique, and single all relate to being one. Lone describes someone or something standing alone. Only means no other person or thing fits that role, stressing exclusion. Unique stresses a special quality nothing else has. Single simply counts one item, not exclusivity.

这四个词都表示“仅此一个”的概念，但强调点不同：lone侧重孤零零、视觉上的孤单，语气偏文学化；only强调排他性，即“再没有别的”；unique强调独一无二、无可比拟的特质；single纯粹指数量上“只有一个”，语气中性。注意：single在其他语境中还有“单身的”之义，only也可作副词“仅仅”，这两个义项这里都不涉及。