it is a mercy that加什么从句

例句：

It is a mercy she was not seriously hurt 

该句子为主语从句！

句中省略that，完整的应是： It's a mercy(that)she was not seriously hurt. 

it 在这里做形式主语，代that she was not seriously hurt将它提前可防止句子头大脚轻。 

正确顺序是:That she was not seriously hurt was a mercy.

又如： It was a mercy that the whole family survived the earthquake.   这次地震中一家人都幸免于难，真是不幸中之大幸.