The last thing I want to know is when I can get it.这句话说得对吗when前用+that吗,为什么

这个句子有多重从句
The last thing(that) I want to know.后面的 I want to know 是The last thing的定语,是个定语从句
The last thing I want to know is (that)when I can get it.后面的when I can get it在这个句子中作表语,是个表语从句,that可以省略