第一句的句子结构是这样的The letter (she looked forward to) came at last.括号中为定语从句修饰The letter由这两个句子可知道look forward to的两种用法,一是look forward to+sth而是look forward to+doing sth...The letter she looked forward to came at last. 能不能改成：She looked forward to the letter which /that came at last.可以的