These words all describe sb/sth that is the largest or most important of its kind. 以上各词均用以形容主要的、最重要的人或事物。

• main [only before noun] largest or most important 指主要的、最重要的：◆Be careful crossing the main road. 过大马路时小心点。◆The main thing is to remain calm. 最重要的是要保持冷静。
• major [usually before noun] very large or important 指主要的、重要的、大的：◆He played a major role in setting up the system. 他在建立这个系统的过程中起了主要的作用。 NOTE Major is most often used after a with a singular noun, or no article with a plural noun. When it is used with the or my/your/his/her/our/their it means 'the largest or most important'. * major 最常用于不定冠词 a 之后，与单数名词连用；或不用冠词，与复数名词连用。major 与 the 或 my/your/his/her/our/their 连用时意为最主要的、最重要的：◆Our major concern here is combatting poverty. 在这里我们最关心的是解决贫穷问题。 In this meaning it is only used to talk about ideas or worries that people have, not physical things, and it is also more formal than main. 在该含义中 major 只用以指人的想法或担忧，而非物质的东西，且较 main 正式：◆Be careful crossing the major road.◆The major thing is to remain calm.
• key [usually before noun] most important; essential 指最重要的、主要的、关键的：◆He was a key figure in the campaign. 他是这场运动的关键人物。 NOTE Key is used most frequently in business and political contexts. It can be used to talk about ideas, or the part that sb plays in a situation, but not physical things. It is slightly more informal than major, especially when used after a noun and linking verb. * key 最常用于商务和政治语境，可用来形容想法或某人在某情势下起的作用，而非物质的东西。key 较 major 稍非正式，用于名词加连系动词后尤其如此：◆Speed is key at this point. 在这个时候速度是关键。
• central (rather formal) most important 指最重要的、首要的、主要的：◆The central issue is that of widespread racism. 最重要的问题是种族主义泛滥。 NOTE Central is used in a similar way to key, but is more formal. It is most frequently used in the phrase sth is central to sth else. * central 与 key 用法相似，但更正式，最常用于 be central to 短语中。
• principal [only before noun] (rather formal) most important 指最重要的、主要的：◆The principal reason for this omission is lack of time. 跳过它的主要原因是时间不足。 NOTE Principal is mostly used for statements of fact about which there can be no argument. To state an opinion, or to try to persuade sb of the facts as you see them, it is more usual to use key or central. * principal 主要用于陈述无可争辩的事实。表明意见或说服别人相信你所见到的事实，较常用 key 或 central：◆The key/central issue here is… 这里的关键问题是…
• chief [only before noun] (rather formal) most important 指最重要的、首要的、主要的：◆Unemployment was the chief cause of poverty. 失业是贫穷的主要原因。
• prime [only before noun] (rather formal) most important; to be considered first 指主要的、首要的：◆My prime concern is to protect my property. 我最关心的是保护自己的财产。

Patterns

• a/the main/major/key/central/principal/chief/prime aim/concern
• a/the main/major/principal road/town/city
• the main/key thing is to…
• to be of major/key/central/prime importance