句子的意识是：我期盼的那封信终于到了.
可以把received改为 arrived
the letter 是主语
I looked forward to 是定语从句 修饰the letter
arrived 是谓语
去掉修饰成分就是：The letter arrived at last.
receive 是及物动词,receive a letter
不知道我有没有解释清楚,可是。。一定要用receive，，怎么改那改为 was received 吧
receive a letter变为被动语态就是 the letter was received