我们知道，在定语从句中，关系词有三个作用：代表先行词；引导定语从句；在从句中做句子成分。当关系代词在定语从句中做主语时，谓语动词用单数还是用复数呢？
　　1. 因为关系代词是代表先行词的，所以当关系代词在定语从句中做主语时，其谓语动词的数应与先行词的数一致。如：
　　（1）The boys who are from America like China.
　　（2）The boy who is playing on the playground is Mrs Smith’s son.
　　（3）The books which are very interesting belong to me.
　　（4）The road that leads to the shore is sandy.
　　2. 如先行词是表示不同概念的两个名词时，定语从句的谓语动词用复数。如：
　　The cabman and the cab that were easily found in London in the past can be seen no more now. （先行词the cabman and the cab，表示“马车夫和出租车”两种不同的概念）
　　3. 在“one of 復数名词 who/that/which引导的定语从句”中，先行词是其中的“复数名词”，定语从句的谓语动词用复数。如：
　　（1）This is one of the most interesting questions that have been asked.
　　（2）He is one of the students who have been to Beijing.
　　（3）Changjiang River is one of the longest rivers in the world which have been known to all of us.
　　4.在“the only/ the very one of the 名词复数 who/that/which引导的定语从句”中，先行词是the only one， the very one，因此，定语从句的谓语动词用单数。如：
　　（1）She was the only one of the girls who was late for the meeting.
　　（2）He is the only one of the students who is elected.
　　（3）Tom is the only one of the students who comes on time every day.
　　[练一练]用括号内所给词的正确形式填空。
　　1. He is one of the students who_________（be） able to answer the questions.
　　2. He is the only one of the students who_______（be） able to answer the questions.
　　3. He is the only one of the students who__________（have）been a winner of scholarship for three years.
　　4. My dad’s got a camera that __________（take） photos under water.
　　参考答案：1. are 2. is 3. has 4. takes
　　总之，定语从句的主谓一致取决于它的先行词，先行词是单数，谓语动词用单数；先行词是复数，谓语动词用复数。但是需要注意“one of 复数名词 who/that/which引导的定语从句”中的谓语动词的数。