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.
