例：
she said she came from china and that she was eighteen.
said后面接有两个宾语从句，第二个从句前面的that不可省。
你说的是这个特殊用法吗？
一般来讲动词或短语后面接的宾语从句前面可以加that也可省略，没什么特别要求。如：
they found out(that)....
she thinks(that)...
但做同位语时that不可省略。如：
i don't know the fact that ....
they've got the news that ...
这里that后面的句子表示news和fact的内容。