句子的 one 是针对前面的 time ,
即 one 代替 situation time ,
原句相等于
The second clause expands the temporal domain by relating a new situation time to the (situation time) that is already in the domain.
你也可以这样说
The second clause expands the temporal domain by relating a new situation time to what is already in the domain.one 只替代 situation time ， 而不是a new situation time， 为什么？one 只替代 situation time ， 而不是a new situation time， 为什么？没有为什么的，这应该是明显的：The second clause expands the temporal domain by relating a new situation time to the one (= situation time) that is already in the domain.如果我说The new book replaces the one that was lost很明显，one 所指的就是 book ，而不是 new book 。这个句子的结构和原句的结构没两样。