it is a mercy that加什麼從句

例句：

It is a mercy she was not seriously hurt 

該句子為主語從句！

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

it 在這裏做形式主語，代that she was not seriously hurt將它提前可防止句子頭大腳輕。 

正確順序是:That she was not seriously hurt was a mercy.

又如： It was a mercy that the whole family survived the earthquake.   這次地震中一家人都倖免於難，真是不幸中之大幸.