六个常用于表示 “主要的”意思的形容词。
main
普通用词，指在一系列事物中，其数量，功能、重要性都大于其他同类事物，通常只用作定语。
1）The main difficulty with the project is that the geological structure of the area is very complicated.
这个项目的主要困难是这个地区的地质构造的非常复杂。
2）At present, vehicle bomb has become the main means of the teeror explosion,which has posed seriously threatened to the lives of many innocent citizens.
汽车炸弹成为当前恐怖爆炸最主要的方式，严重威胁倒了许多无辜公民的生命的安全。
3）There are several reasons for this problem. One of the main reasons is that the number of vehicles is increasing much more rapidly than building of roads.
关于这个问题，有很多原因。一个主要原因是车辆增加的数量远快于道路的建设。
4）Money was the main factor in my decision not to buy a car.
我决定不买车主要是因为钱。
major
侧重指与同类其他事物相比更重大、更突出。
1）Major crops in these provinces are wheat.peanuts, corn and soy beans.
这些省份主要的作物是小麦、板栗、玉米和大豆类。
2）This new program will make a major step forward in the spreading the Olympic ideals.
这项新计划将在传布奥林匹克理想迈出重大的一步。
3）Hence, a major content of the reform is to constantly enhance the regulatory role of the market in the economy .
因此，改革的一项主要内容是不断增强市场对经济的调节功能。
4）Our major planning items contain estimating of cost and construction schedule.
我们主要的计划工作项目包括费用预算和施工进度.
chief
用于人时，表同类中职位最高，权力最大；用于物时，表示同类中最重要，价值最高。故含凌驾他人或他物之意。
1）Her husband was the chief beneficiary of her will.
她丈夫是她遗嘱的主要受益人。
2）For thousands of years, drugs and surgery have proved two of the chief methods of treating diseases.
数千年来，药物和手术提供了两种主要的治病方法。
3）Financial stress is well established as a chief reason for divorce.
经济压力被确认是导致离婚的首要原因。
4）Industry and modesty are the chief factors of his success.
勤奋和谦虚是他成功的主要因素.
primary
通常不用于人。用于物时，指在重要性方面占主导位置。
1）The sun is the primary body in our neighborhood.
太阳是我们的宇宙邻居中的主要天体。
2）Motor vehicles are the primary source of several important air pollutants:carbon monoxide, volatile organic compounds, and nitrogen oxides.
机动车辆是几种重要的空气污染物的主要来源：一氧化碳、挥发性有机化合物,和氮氧化物。
3）On devices such as mobile phones and palmtops, menus must often be relied upon as a primary command vector.
在手机以及其他掌上设备上，菜单是一种主要的命令途径。
4）The primary purpose of education is not to teach you to earn your bread, but to make every mouthful swwter.
教育的主要的目不是教你如何挣得面包，而是使每一口都更加香甜。
principal
也可指优于他物而显得重要，但口气略比 primary 轻。
1）Today, Buddism and Islam are the principal religions of the most parts of Asia.
今天，佛教和伊斯兰教是亚洲大部分地区的主要宗教。
2）Indeed, stress is emerging as one of the principal contributors to poor helath in modern countries.
的确，压力在现代化国家中是造成健康不佳的主要原因。
3）My wages are the principal source of my income.
薪金是我收入的主要来源。
4）The principal thing in this world is to keep one’s soul aloft. ——Flaubert,French writer
人生在世首要大事是保持灵魂的高尚。 ——法国作家福楼拜
prime
虽常用表示“主要的”意思，但强调是“首要的”。
1）The quality of housing, considered the prime indicator of living conditions,leaves much to be desired.
住房质量的被认为是居住条件主要的指标，仍有许多地方有待提高。
2）Our market analysis tells us our prime users will be between 40 to 60.
我们的市场分析告诉我们，我们产品主要的使用者年纪将在40至60岁。
3）The new government’s prime task is to reduce the level of inflation.
新政府的主要任务是减低通货膨胀的水平。
4）The prime difference between innovators and others is one of approach.
创新者与普通人的主要区别在于处理问题的方法不同。
来源：http://blog.sina.com.cn/s/blog_7cbb94610102vwcp.html