firstly

Compared meaning: used to introduce the first point or action (adverb).

比较义项：首先（用于引出最先做的事或列举的第一点）（副词）。

• first

  Neutral, everyday word for beginning a sequence of actions or points; common in both speech and writing, and can introduce a real-world action, not just a list item.

  “first”是日常、中性的说法，用来引出一连串动作或要点中的第一步；口语和书面语中都很常见，既可以引出实际的动作，也可以引出列举中的第一点。

  First, turn off the power before you open the machine.

  首先，打开机器之前要先关掉电源。

• firstly 本词

  More formal; used mainly in writing or formal speech to open a numbered list of points, typically followed by secondly, thirdly; rarely used for a real physical action.

  “firstly”更为正式，主要用于书面语或正式演讲中，用来引出分点列举的第一点，后面常接“secondly”“thirdly”；很少用来引出实际的动作。

  Firstly, the plan is too expensive; secondly, it takes too long.

  第一，这个计划太贵了；第二，耗时太长。

Both words mean introducing what comes first, but formality differs. First is everyday, used for real actions and listing points (its adjective sense, as in the first person to arrive, is not compared here). Firstly is formal, used in writing to open a list.

这两个词在“首先”这一义项上意思相近，容易混淆，但语气和使用场合不同：“first”更口语化，既可用于描述实际行动的先后顺序，也可用于列举要点；“firstly”更正式，多见于书面或正式演讲中列举要点，很少用来描述实际动作。此外，“first”还有序数词/形容词用法（如“the first person to arrive”，意为“第一个到达的人”），这里不作比较。