连接副词的概念：

连接副词可分为两类，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.

连接副词的分类：

连接副词可分为两类，一类是用于连接句子或从句，常见的有therefore, besides, otherwise, however, moreover, still, thus, meanwhile等；
另一类是用于引导从句或不定式，主要的有when, why, where, how等。

连接句子或从句的连接副词：

其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：
如：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。

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后不能接不定式，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.

表语从句的概念：

用作表语的从句叫作表语从句，它位于主句中的连系动词之后。引导表语从句的词有
从属连词that、whether、as though(if)；
关系代词who, what, which, whom, whose, whatever, whoever, whomever, whichever等；
关系副词when, where, why, how, however, when。

表语从句用法：

1、表语从句的引导词：
引导表语从句的词有连词that, whether，连接代词和连接副词，关系代词型what，以及as if,as though,because等连词：
如：The fact is that he doesn't really try. 事实是他没有做真正的努力。
        The question is whether the film is worth seeing. 问题是这部电影是否值得看。
        The problem is how we can find him. 问题是我们如何找到他。
        That was when I was fifteen. 这是我15岁时发生的事。 
        It isn't as if you were going away for ever. 又不是你离开不回来了。
        It is because I love you too much. 那是因为我太爱你了。
注：whether可此导表语从句，但与之同义的if 却通常不用于引导表语从句；because可引导表语从句，但与之同义的since, as, for等也不用于引导表语从句。

2、连词that的省略问题引导表语从句的that通常不省略，但在口语或非正式文体中有时也可省略：
如：My idea is(that) we should do it right away. 我的意见是我们应该马上干。
        The trouble is(that) he is ill. 糟糕的

表语从句用法拓展：

名词性从句在be等系动词后作表语时被称为表语从句，表语从句只能置于主句之后，而主句的动词只能是联系动词。
例如：The problem is how we can get the things we need. 问题是我们怎样能弄到我们需要的东西。(how在表语从句中充当方式状语) 　　
            The scissors are not what I need. 这把剪刀不是我所需要的。(what在表语从句中充当宾语) 　　
            What I told him was that I would find him a good play.
            我告诉他的是我会给他找个好剧本。(what在主语从句中作直接宾语，that作为表语从句的引导词在该表语从句中不充当句子成分，不能省略) 　　
            That is what I want to tell you. 那就是我想要对你讲的。(what在表语从句中充当直接宾语) 　　
            That is why she failed to pass the exam. 那就是她考试不及格的原因。(why在表语从句中充当原因状语)
注意：“That is why...”是常用句型，意为“这就是……的原因/因此……”，其中why引导的名词性从句在句中作表语，该句型通常用于针对前面已经说明过的原因进行总结：
如：That is why you see this old woman before you know, Jeanne.珍妮，这就是现在这个老太婆出现在你面前的原因。(前文提到Jeanne对老妇人显得苍老憔悴深感诧异，说话人对她讲述了其中的原因之后，用这一句来进行概括)。//ThatiswhyIcame.这就是我来的原因。下面是两个与“That is why...”形式相似的结构，它们与“That is why...”结构之间的关系要能够辨析清楚：　　
（1）“That is why...”与“That is the reason why...”同义，只不过从语法结构上讲，“That is the reason why...”中why引导的是—个定语从句，将其中的the reason去掉则与“That is why...”结构一样：
如：That is(the reason) why I can not agree. 这就是我不能同意的理由。　　
（2）“That is because...”句型中从属连词because引导的名词性从句在此作表语，这也是个常用句型，意为“这就是为什么……/因为……”。“That is because...”与“That is why...”之间的不同在于“That is because...”指原因或理由，“That is why...”则指由于各种原因所造成的后果：
如：He did not see the film last night.
        That is because he had to help his little sister with her homework. 昨天晚上他没有去看电影，那是因为他得帮助他的妹妹做作业。(第一句话说明结果，第二句话说明原因) 　　          
        He had seen the film before. That is why he did not see it last night. 他以前曾看过那部电影，因此他昨天晚上没有去看。(第一句话说明原因，第二句话说明结果)