单数名词为何用复数形式的物主代词来指代

问题描述

《剑桥高阶英汉双解词典》 attention 条的一个例句：
A good teacher is always attentive to their students' needs. 一个好的老师总是对学生的需要很关心。
对于句中的物主代词 their 我不是很理解。我觉得 A good teacher 应该对应单数物主代司 his，而不应是 复数形式的物主代词 their，请专家指教，谢谢！

老师答疑

曾老师：

你提供的词典例句是正确的，并不是笔误。关于这个问题，Michael Swan《牛津英语用法指南》有具体说明：

they / them / their 常常用来指单数不确定的人，常常用在 a person, anybody/one, somebody/one, nobody/one, whoever, each, every, either, neither和no后面。在这种情况下，they用复数动词。

If a person doesn't want to go on living, they are often very difficult to help. 一个人要是不想活下去的话，常常令人爱莫能助。

If anybody calls,take their name and ask them to call again later.如果有人来电话，就把他的姓名记下来，同时请他迟些再打电话来。

Nobody was late, were they? 没有人迟到吧？

Whoever comes, tell them I'm not in. 不管谁来，告诉他我不在。

......

当所指的人可能是男性也可能是女性时（正如上面所举的例子），they / them / their 的这一单数用法用起来很方便。用he or she, him or her 以及his or her会很累赘，尤其是在需要重复使用的时候。很多人在这情况下不喜欢沿用以前的用法，即用he / him / his。

然而，they / them / their 也可以用于性别明确的场合。下面举几个采访中的例子：

I swear more when I'm talking to a boy, because I'm not afraid of shocking them. 我跟男孩子谈话，爱说脏话，因为我不怕吓着他们。

No girl should have to wear school uniform, because it makes them look like a sack of potatoes. 女孩子不一定非穿校服不可， 因为校服使她们看起来像一大口袋土豆。

they / them / their 有时用来指一个具体而又不确定的人。

I had a friend in Paris, and they had to go to hospital for a month. 在巴黎我有一位朋友，他得去医院看一个月的病。