可分为两类，一类是用于连接句子或从句，常见的有therefore, otherwise, however, moreover, still, thus, meanwhile等；
另一类是用于引导从句或不定式，主要有when, why, where, how等。
例如：We all tried our best; however, we lost the game.
Tell me when we shall leave.
I do not know how to find him.
连接副词的特点：
一、连接句子或从句的连接副词
连接副词的性质类似于并列连词，使用时其前通常用分号或句号；
若其前用逗号，则通常带有并列连词(如and)。如：
I don't like it; besides, it's too expensive. 我不喜欢它，而且也太贵了。
We all tried our best; however, we lost the game. / We all tried our best. However, we lost the game. 我们都已尽了最大的努力，不过我们还是输了。
注意，有的连接副词(如however等)后通常有逗号与句子的其他成分隔开。另外，有的这类副词还可位于句中或句末：
Peter is our youngest child, and we have three others besides. 彼昨是我们最小的孩子，我们另外还有三个孩子。
He may, however, come later. 不过，他也许一会儿就到。
We all tried out best. We lost the game, however. 我们都已尽了最大的努力，不过我们还是输了。
二、引导从句和不定式的连接副词
用于引导从句(名词性从句)或不定式的连接副词主要有when, why, where, how等。如：
Tell me when we shall leave. / Tell me when to leave. 告诉我什么时候离开。
I don't know how I can find him. / I don't know how to find him. 我不知道如何找到他。
Where we can get the money is just our problem. / Where to get the money is just our problem. 到哪里去弄到这笔钱正是我们头痛的事。
That's why he speaks English so well. 那就是他为什么英语讲得这么好的原因。
注：连接副词why 后不能不定式，如可说 I don't know why I must leave.(我不知道我为什么必须离开)，但不能说 I don't know why to leave。
连接副词的句法特征：
所谓连接代词和副词，实际上就是我们以前学过的疑问代词和副词，如who, whose, whom, what, which, when,where, why,how以及由how构成的词组。
1.连接副词起对等连接词的作用,连接两个句子或子句,如:
however, so, then,moreover,concequencely,hence,also,besides,,otherwise,therefore,thus等。
I feel a bit tired. However, I can hold on.我有点累了,但我能坚持下去。
They talked about it for hours. Finally, they decided to go。他们对此商量了好几个小时,最后他们决定去。
2.连接副词通常位于子句的句首,有的也可位于句中。
They talked about it for hours. Finally, they decided to go。他们对此商量了好几个小时,最后他们决定去。
finally作为连接副词,放在句子的句首。
I have never been to Wales. I, therefore, don't know much about it.我从未去过威尔斯,因此我对它知道的不多。
therefore作为连接副词,位于句子的句中。
3.有些连接副词的含义彼此相同或相近,在使用时可以互相替换,而不会改变原句的含义。
The weather was cold. Therefore(Thus), we stayed home.天气寒冷,因此我们待在家里。
therefore和thus的含义相近,因而也可以互相替换。
在应用时要注意以下两点：
一是它们在构成疑问句时叫疑问词，引导宾语从句时称作引导词。
一是它们和that不一样，在从句中有含义，担任相关的句子成分，所以不能省略，另外，它们须放在宾语从句之首，宾语从句必须是陈述句结构。
引导名词性从句的连接词可分为三类：
连接词：that, whether, if, as if, as though（不充当从句的任何成分，但绝不能省略）
连接代词：what, whatever, who, whoever, whom, whomever, whose, which, whichever.
连接副词：when, whenever, where, wherever, how, however, why
不可省略的连词：
1. 介词后的连词
2. 引导主语从句和同位语从句的连词不可省略。
That she was chosen made us very happy.
We heard the news that our team had won.
大部分连接词引导的主语从句都可以置于句末，用 it充当形式主语。
It is not important who will go.
It is still unknown which team will win the match.
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