1.连接副词起连接两个句子或子句的作用如:however,moreover,consequently,hence,also,besides,,otherwise,therefore,thus,thereupon等等

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。
　　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。
　　The weather was cold. Therefore(Thus), we stayed home.
　　天气寒冷,因此我们待在家里。
　　 2.连接副词通常位于子句的句首,有时也可位于句中。

（1）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。

（2）I have never been to Wales. I, therefore, don't know much about it.

我从未去过威尔士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。

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。

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.

比较：whether与if 均为"是否"的意思。 但在下列情况下，whether 不能被if 取代：

1. whether引导主语从句并在句首

2. 引导表语从句

3. whether从句作介词宾语

4. 从句后有"or not"

Whether he will come is not clear.