()he has worked in the school for several years is known to everyone.

问题描述:

()he has worked in the school for several years is known to everyone.
为什么要填that,而不是what,不是缺主语吗?

因为he has worked in the school for several years 这个主语从句意思完整,不缺成分
所以才选没有实际意义的That