爱华网本文转载，自http://kmxuepeng.blogbus.com/logs/73502204.html薛鹏的blog他的博客中有很多英语写作关键词汇用法，感觉很有用。
学生写作时总会犯各种并无大碍的小错，但见多了之后，就觉得心烦，总想把其中道理讲个明白。这次处理一个小词：however。学生作文中频繁出现这样的句子（截屏）：
Office Word已经标示出了句子错误所在，即绿色波浪线那部分。点击右键，软件会提供改进建议：将degradation后面的逗号更换为分号。调整以后，分号前后各是一个独立的句子。这与调整前有何不同？之前，however居在两句中间，貌似起到了连接两个句子的功能。调整后，however便不再扮演连接的角色。这有道理吗？翻阅词典便知道，however作“然而、但是”讲时只是个副词，副词怎能连接两个句子呢？所以，Word给出的改进建议是对的（但有极个别副词可以连接句子：IcamedirectlyIgot your message.）。故请牢记：
表示转折关系时，however是副词。一个句子写完之后标句号或分号，其后才可以写however。比如：
These calculations suggest that a period of about one million yearswould have been required.However,other reliable evidencesuggests that the deposition of the boundary clay could not havetaken one million years. [TPO8EXTINCTIONOF THE DINOSAURS]
清楚了这一点就可以写对了。但既然已动笔探究这个词儿，就不妨再多研究几个小问题（若你不关心however更进一步的用法，读到这里也可以啊）。首先，上述句子中，副词however到底充当什么成分？
However后面是一个完整的主谓宾结构，两者由逗号隔开，且however对后面句子的意思没有影响（有人说这不是“但是”的意思吗？对，但这不干涉后面句子的意思，“但是”这个意思影响的是前后两个句子的逻辑关系。）。这种情况就是插入语。插入语就是一个被逗号与句子主干隔开、不影响句子本身意思的词、短语或从句，它体现的往往是句子之间的逻辑关系、说话人的感情色彩、额外补充的信息等。插入语可以在句首，如上面的however，当然也可以放在句尾，比如：
The question of ecosystem stability is complicated,however.The first problem is thatecologists do not all agree what “stability” means. Stability canbe defined as simply lack of change. [TPO3TheLong-Term Stability of Ecosystems]
那么，插入语能否在句子中间呢？当然可以，且至少对于however而言大部分情况下都是在句子中间的。粗略统计刘文勇所编辑的黄金阅读6.0版本（由于文章下面还有试题，故某些文字会重复，本次统计没有排除重复的情况，故曰“粗略”），单独搜索“however”结果为122处（其中1处为刘文勇自己所做英文前言里的，下同），搜索“, however,”结果为80处，可见多数情况下however是用在句子中间的。
可新的问题出现了：“句子中间”这个位置有多种可能性，到底哪儿最好？这80处中，however有些在主语后，有些在谓语动词后，又有些在状语后面，无章法可循。其实，在这纷繁复杂中至少有一种情况可以掌握，也需要学生掌握。在说明之前，需要先澄清这么一回事：
“然而、但是”表示的是转折关系，可转折又分为两种情况。一种是让步（相当于汉语的“虽然……但是……”和“尽管……然而……”），另一种是对比（甲怎么样，然而乙却怎么样）。且看两个简单的例句：
①My uncle has a huge collection of books; however,he never reads a single one.
②After graduation, some of my classmates studiedfor master degrees; however, others got jobs and started theircareers.
上述句子中，①中however体现的是让步，②中however体现的是对比。这个弄明白了之后，就可以回到刚才的问题上面来了：however到底摆在句中何处？刚才所言的“可以掌握、需要掌握”的情况是：在对比关系中，为了凸显对比的对象，可在第二个对比对象后插入however。比如：
In America, the school day ends at 2:30 pm, followed by threehours’ sports; in China,however,students have to worktill 9:30 pm.
由于是在中美之间做对比，所以however置于in China后凸显对比对象。照此道理，句子②就可以修改为：
After graduation, some of my classmates studied for master degrees;others,however,got jobs and startedtheir careers.
再看两个新托福阅读中的例子吧：
③About 400 million years ago, when the present-daycontinents of North America and Europe were joined, the Caledonianmountain chain was the same size as the modern Himalayas.Today,however,the relics of theCaledonian orogeny (mountain-building period) exist as thecomparatively low mountains of Greenland, the northern Appalachiansin the United States, the Scottish Highlands, and the Norwegiancoastal plateau. [Official GuideGEOLOGYAND LANDSCAPE]
④Many have been attempting to conserve water byirrigating less frequently or by switching to crops that requireless water. Other,however,have adopted thephilosophy that it is best to use the water while it is stilleconomically profitable to do so and to concentrate on high-valuecrops such as cotton. [TPO3Depletionof the Ogallala Aquifer]
不难理解，③中对比的是“about 400 million years ago”和“now”；④中对比的是“many”和“others”。
