收到某人的开信，4种表达方式如下：
1.Received the letter from somebody
2.Somebody's letter was received by me
3.I received somebody's letter
4.Somebody sent the letter to me
四种句式变换，主要通过哪个来做主语实现变换，第一个句子是收信人做主语，第二个句子是信做主语，第三个句子是收信人做主语，但是信加了修饰词，第四句是寄信人做主语
