They redo what they failed to do.这里的what 能换成that吗?很多宾语从句或主语从句用what连接,我也感觉应该用what.但是,说不出原因,为什么用what而不用that.

They redo what they failed to do.这里的what 能换成that吗?很多宾语从句或主语从句用what连接,我也感觉应该用what.但是,说不出原因,为什么用what而不用that. what they what()they()they()there?what()they()they()there?[do] what do they do they They shall do what they what do they what do they do? what are they What do they what do they what did they What are they? What do they need? What are they doing? what are they doing? what animals do they they/have/what/do/(?) what nice they are