The trouble is that I have lost his address. 麻烦是我把他的地址丢了。
(2) 从属连词whether, as, as if。如：
He looked just as he had looked ten years before. 他看起来还与十年前一样。
The question is whether they will be able to help us. 问题是他们是否能帮我们。
All this was over twenty years ago, but it's as if it was only yesterday.
能跟表语从句的谓语动词一般为系动词be, seem, look等。如：
It looked as if it was going to rain. 看起来天要下雨了。
(3)连接代词who, whom, whose, what, which, whoever, whatever, whichever
连接副词 where, when, how, why。
如：The problem is who we can get to replace her. 问题是我们能找到谁去替换她呢。
The question is how he did it. 问题是他是如何做此事的。
That was what she did this morning on reaching the attic. 那就是她今晨上了阁楼干的。
I think it is because you are doing too much. 我想这是因为你做得太多。
My suggestion is that we (should) start early tomorrow. 我的建议是我们明天一早就出发。
(1) 从属连词that。 如：That they were in truth sisters was clear from the facial resemblance between them.
Whether he’ll come here isn’t clear. 他是否会来这里还不清楚。