principally

Compared meaning: mainly, for the most part (adverb).

比较义项：主要地、大部分地（副词）。

• primarily

  Primarily highlights the main purpose or most fundamental reason among several, often describing what something is basically for; neutral register, common in both speech and writing.

  primarily强调若干原因或目的中最主要、最根本的一个，常用来说明某事物的根本用途；语域中性，口语和书面语都常用。

  The course is aimed primarily at beginners, though some material suits advanced learners too.

  这门课程主要面向初学者，不过也有一些内容适合进阶学习者。

• mainly

  Mainly is the plain, everyday word for 'for the most part'; it describes the largest share or the most common case, used casually in both conversation and writing.

  mainly是表示“大部分、主要”最普通、最常用的词，用来描述占比最大或最常见的情况，口语和书面语中都随意使用。

  We mainly eat vegetables at home, but we order pizza on Fridays.

  我们在家主要吃蔬菜，不过星期五会叫披萨。

• principally 本词

  Principally is a formal near-synonym of primarily, used in reports, business or official writing to name the chief factor or purpose; it rarely appears in casual talk.

  principally是primarily的正式近义词，用于报告、商务或公文等书面语中，表示主要因素或目的，很少用于日常口语。

  The report blames the crash principally on driver fatigue rather than mechanical failure.

  报告认为，这起事故主要是由司机疲劳驾驶造成的，而非机械故障。

• chiefly

  Chiefly is a formal, somewhat literary word most often paired with a reason clause, or used to single out one dominant fact among several; uncommon in casual speech.

  chiefly是较为正式、略带文学色彩的词，最常与表示原因的从句连用，或用来突出若干事实中最主要的一个；口语中不常见。

  She is remembered chiefly for her work on early computer programming.

  人们记住她，主要是因为她在早期计算机编程方面的工作。

Mainly, primarily, principally, and chiefly all mean 'mostly'. Mainly is the most common, casual word. Primarily points to the main reason among several. Principally is formal, used in reports or official writing. Chiefly is formal, often paired with a reason, or naming the main fact.

这四个词在“主要地”这一意义上非常接近，但语域和搭配不同：mainly最常用、最口语化；primarily强调若干原因或目的中最根本的一个；principally和chiefly更正式，principally常见于报告和公文，chiefly常与表示原因的从句搭配，或用来突出若干事实中最主要的一个，语气略显文雅。