关系代词在英语中非常重要，可以引导定语从句，连接主句和从句。五个常用的关系代词是who、whom、whose、which和that。Who和that可以代替人或物，作为主语或宾语；whom只能作为宾语代替人；whose表示所属关系；which代替物品或事情。这些关系代词可以帮助我们更准确地表达意思。
1. Who
Who通常用来代替人，作为主语或者宾语出现在定语从句中。
- My sister, who is a doctor, works in a hospital. (我的姐姐是位医生，她在医院工作。)
- The man who I met yesterday is my neighbor. (我昨天遇到的那个人是我的邻居。)
2. Whom
Whom同样用来代替人，但它只能作为宾语出现在定语从句中。
- The woman whom I saw at the party is my friend. (我在聚会上见到的那个女人是我的朋友。)
- The teacher whom I asked for help is very kind. (我向他求助的老师非常友善。)
3. Whose
Whose用来表示所属关系，它后面通常跟着名词。
- The boy whose bike was stolen reported it to the police. (自行车被偷的那个男孩向警察报案了。)
- The manager whose team won the championship was very proud. (带领球队获得冠军的经理非常自豪。)
4. Which
Which用来代替物品或事情，作为主语或者宾语出现在定语从句中。
- The book which I borrowed from the library is very interesting. (我从图书馆借来的那本书非常有趣。)
- The car which he bought last year is very expensive. (他去年买的那辆车非常昂贵。)
5. That
That既可以代替人，也可以代替物品或事情，作为主语或者宾语出现在定语从句中。
- The dog that is barking is mine. (正在叫的那只狗是我的。)
- The restaurant that we went to last night is very popular. (我们昨晚去的那家餐厅非常受欢迎。)
