that什么情况可以放在句首？

代词，表示那个，
That boy is tall.

作为主语从句的连词，
That he has passed the exam is really a good news.
他通过了考试，真是个好消息。

就是这些，

作为主语从句的连词，
That he has passed the exam is really a good news.
他通过了考试，真是个好消息。