1. 用as引导
非限制性定语从句的:
As we all know;As everyone/everybody knows;
As is known to us all;As is known to everyone/everybody
2. 用It做形式主语,that从句做真正主语的:
It is known to us all that从句;
It is known to everyone/everybody that从句
3. 用what引导
主语从句的,后用that引导
表语从句的:
What is known to us all is that从句;
What is known to everyone/everybody is that从句
参考资料:英语牛人团