fast

Compared meaning: happening or moving quickly (adjective).

比较义项：快速的；迅速的（形容词）。

• fast 本词

  The most general word for speed; it can describe people, vehicles, actions or events, and combines freely with almost any noun without restriction.

  表示“快”时最通用的词，可以形容人、车辆、动作或事件，几乎能与任何名词自由搭配，没有特殊限制。

  A fast car overtook us on the highway just before the exit.

  一辆速度很快的汽车在高速公路出口前超过了我们。

• express

  Used almost only right before certain nouns such as train, delivery or service, meaning a special option faster than the usual one; not used to describe people directly.

  几乎只用在列车、快递、服务等特定名词前，表示比普通选项更快的专项服务；一般不直接用来形容人。

  The express train from the airport reaches downtown in just fifteen minutes.

  从机场出发的特快列车只需十五分钟就能到达市中心。

• quick

  Focuses on an action or event taking little time rather than on pure speed; very common in informal, everyday contexts such as replies or short visits.

  强调动作或事件耗时短，而不是单纯速度快；常见于非正式的日常场合，比如回答或短暂的拜访。

  She gave a quick answer before rushing off to catch her bus.

  她迅速回答了一下，就匆忙跑去赶公交车了。

• prompt

  Emphasizes acting without delay or hesitation, especially when responding to something; typical of formal or business contexts, such as replies or payments.

  强调不拖延、不犹豫地行动，尤其是在回应某事时；常见于正式或商务场合，比如回复或付款。

  The company was known for its prompt replies to every customer complaint.

  这家公司以对每一条客户投诉都能及时回复而闻名。

All four words can mean fast. Fast is the most general word, for people, vehicles, or actions. Quick focuses on an action taking little time. Express goes only before words like train or delivery, not people. Prompt means acting without delay, especially in replies.

这几个词都可表示“快”，但侧重不同：fast 最通用，可形容人、车辆或动作；quick 强调动作耗时短，多用于口语；express 只用在列车、快递等特定名词前表示专项快速服务，不能形容人；prompt 强调不拖延、及时回应，语气偏正式。这里不比较 express 表示“表达（感情）”的动词用法，也不比较 fast 表示“斋戒”的动词用法。