rapid

Compared meaning: fast in speed (adjective).

比较义项：快速的（形容词）。

• rapid 本词

  Describes fast change, growth or movement, especially over time; often modifies abstract nouns like growth, decline or heartbeat, and sounds formal; rarely describes a person's own actions.

  形容快速的变化、增长或移动，常修饰增长、下降、心跳等抽象名词，语气较正式，很少直接形容人本身的动作。

  The country's rapid economic growth surprised international observers.

  这个国家迅速的经济增长让国际观察家感到惊讶。

• fast

  The general, everyday word for quick speed; describes people, vehicles or actions directly, and also works as an adverb, making it the default, neutral choice.

  是表示速度快最常用、最日常的词，可以直接形容人、交通工具或动作，也能作副词使用，是中性、默认的选择。

  She is a fast runner and always wins the school races.

  她是一个快速的短跑运动员，总是赢得学校的比赛。

• express

  Used almost only in fixed phrases for transport or services that skip stops or delays, such as an express train or express delivery; not used freely to describe general speed.

  几乎只用于固定搭配，指跳过中途站点或环节、不延误的交通工具或服务，例如快车、快递；不能自由地用来形容一般的速度。

  The express train from Beijing to Shanghai skips every small station.

  从北京到上海的快车会跳过每一个小站。

• quick

  Emphasizes a short duration or promptness of a single, brief action, often mental — like a decision, glance or reply — rather than high physical speed.

  强调一个简短动作耗时短、反应迅速，常用于思考、决定、瞥一眼等脑力或简短行为，而不是形容物理速度快。

  He gave a quick reply before hanging up the phone.

  他挂电话前给了一个快速的回答。

Rapid, fast, express, and quick all mean fast, but differ. Rapid is formal, for abstract change like growth. Fast is the everyday word. Express appears only in fixed phrases. Quick means a short, often mental action. Not covered: express as a verb or noun.

这四个词都可以翻译成“快”，但用法和搭配不同：rapid 偏正式，常修饰抽象名词；fast 是最常用、最中性的说法；express 几乎只用于固定搭配（如快车、快递）；quick 强调动作耗时短、常用于脑力活动。这里不比较 express 作动词“表达”和作名词“快递”的意思。