1.  It was thought that he was waiting there now.

正确。now在讲故事的时候可以指故事中的某个特定时间。waiting和thought为同一时间，waiting未结束，故用过去进行时态。

2.  It was said that he was working very hard.

正确。work和say为同一时间，work未结束，故用过去进行时。

3.  It was said that a lot of electrical appliances were stolen every day.

正确。被偷是习惯性动作，与说为同一时间，故都用一般过去时。

4.  It is understood that he had disliked children.

错误。主句为一般现在时，从句的过去完成时属于错误。dislike为静态动词，从句没有过去时间状语，应该理解dislike与主句谓语为同一时间，故从句应改为一般现在时dislikes。如果上下文暗示一个过去时间，则从句谓语也可用一般过去时disliked。

5.  It is expected that the rate of exchange will go down soon. 可以用下面的句子表示吗？The rate of exchange is expected to be going to go down soon. 

to be going to go两个go用在一起很别扭。因此正确的应该是：

The rate of exchange is expected to go down soon.