老师您好！我有两个问题：

He is the only one of the students who have been abroad.

  1.这句话中的have是不是需要改为has?

  2.为什么在上面的定语从句中要用who而不是that？（原因是：英语语法关于定语从句中讲到，先行词被the only修饰的话，只能用that来引导定语从句，而不能用who或其他引导词引导.）

回答如下

(1)是需要改为单数，因为，关系代词who指代先行词为the only one不是the students.

This is the only one of the best novels that has appeared this  year.

(2)如果先行词为the only one，如果指事物用关系代词that 引导定语从句，如果指人，用who 引导定语从句。

He was the only one of the  boys who wasgiven a prize.

(1)the only one of the +名词复数，后面关系引导定语从句谓语用单数。因为定语从句主语为“the only one” 因此谓语用复数.

Mary is the only one of the youngest girls who plays in the band.

玛丽是参加乐队伴奏的最年轻的姑娘。

(2)one of the +名词复数，后面关系引导定语从句谓语用复数,因为定语从句主语为“名词复数” 因此谓语用复数.

This is oneof the rooms that were damaged in the fire.

这就是那次大火烧毁的房间之一。

继续跟踪回答如下：

这位同学看得出来，对知识很具有钻研精神，这点非常好的！关于你的这个问题，可能你的老师没有给你讲清楚，当先行词被the very,the only,the last, 以及序数词，形容词最高级修饰时，要用关系代词that引导定语从句时针对于关系代词which来讲的，不是关系代词who,而且先行词指代代替事物的条件

只能用关系代词that 引导定语从句不能用关系代词which,条件如下

关系代词指代某物事时，只用that不用which的情况

最大条件关系代词所代替的名词只能是表示“事物”的名词 或者“人+事物”的名词；其它条件如下：

(1)当先行词是everything, anything, nothing 。 all, none, few, little, some等不定代词时，或当先行词受every, any, all, some, no, little, few, much等词修饰时。

Is there anything that I can do for you?

Do you have anything that you say to me ?

(2)先行词被序数词或形容词最高级修饰时。

This is the most beautiful place that I have ever visited.

(3)先行词被the very，the only，the right，the first，the last等修饰时

This is the last thing that I want to tell you.

(4)先行词前有the same 修饰时

This is the same pen that I lost last night.

(5)如果表示和先行词是同一物时用the same…that…

This is the same purse that I lost yesterday.

这就是我昨天丢的那个钱夹。

(6)如果表示与先行词同一样的某物

This is the same purse that I lost yesterday.

这个钱夹和我昨天丢的那个一样。

(7)先行词是并列结构，既有人又有物时

He can remember the things and the persons that he met on his trip.

(8)先行词为数词时

Look at the books on the bookshelf. You can see the two that you bought me for my birthday.

(8)避免重复原则，当先行词前面有who, which等疑问代词时

Which is the pen that you need?

(9)关系代词在定语从句中作表语时，而且通常省略

My hometown is no longer the place that it used to be.

老师，您好！您讲的非常详实，谢谢!

   老师，您举的例子都是指代事物的，此时不用关系代词which，这一点，我理解了。但是，对于以上这些情况，如果是指代人的时候，是否要用关系代词who呢？

   另外，麻烦老师给推荐一本英语语法书，希望能够尽量全面一些的，大学水平的。

你好！目前市场英语语法书都不很理想，相互抄袭现象非常严重，我的英语是自学的，都是很多年前购买数百本语法专著从自学的，如果可以，可以加我QQ2248461201

关于你的问题

the very/the only/the last/the first +表示人的名词，后面that/who都可以，只是用WHO更好些

He is the very person that/who I am looking for.

the very/the only/the last/the first +表示人的不代词 或者先行词为表人的不定代词，则需要用WHO 关系代词.

There is someone who wants to see you.