the only thing that you should do is to try your best.语法有问题么?不知道语法上有没有问题!如果有的话 请指正!我把the only thing that you should do 当成主语从句!

the only thing that you should do is to try your best.语法有问题么?不知道语法上有没有问题!如果有的话 请指正!我把the only thing that you should do 当成主语从句!
the only thing that you should do is to try your best.语法有问题么?
不知道语法上有没有问题!如果有的话 请指正!
我把the only thing that you should do 当成主语从句!

the only thing that you should do is to try your best.语法有问题么?不知道语法上有没有问题!如果有的话 请指正!我把the only thing that you should do 当成主语从句!
语法上没有问题,其中to可以省略.
该句也为定语从句,先行词为thing

对的

that是多余的，当然加了也不错

the thing you only should do is to try your best这样才对

没错，不过不是主语从句。 主语是the only thing，后接that引导的定语从句