本来就有单数词性的：anybody, anyone, everybody, everyone, nobody, no one, somebody, someone。
问题在于在下文使用代词时无法确定使用he还是she。
目前：
-在口语中，可用they指代anybody, everybody, somebody, etc.
-在正式书面语中：
（1）如可确定性别，则用he/him 或是 she/her
（2）如不确定性别，则用 "he or she"
（3）重写句子以避免此问题：
·变单数为复数：When someone studies, he can learn a lot. --> When people study, they can learn a lot.
·用you改写句子：When you study, you can learn a lot.
·去掉代词：Studying is a good way to learn a lot.
