原文

1. Form the possessive singular of nouns with 's.
单数名词的所有格在后面加’s

Follow this rule whatever the final consonant. Thus write,
无论单词以什么辅音字母结尾,都要遵守这条规则,写成:

Charles's friend
Burns's poems
the witch's malice

This is the usage of the United States Government Printing Office and of the Oxford University Press.
美国政府印刷局和牛津大学出版社都遵循这一规则。

Exceptions are the possessives of ancient proper names in -es and -is, the possessive Jesus', and such forms as for conscience' sake, for righteousness' sake. But such forms as Achilles' heel, Moses' laws, Isis' temple are commonly replaced by
例外包括以es或is结尾的古代专有名词,耶稣的所有格Jesus’,以及for conscience’ sake,for righteousness’ sake这类型短语。然而,Achilles' heel,Moses' laws,Isis' temple等短语通常写成以下形式:

the heel of Achilles
the laws of Moses
the temple of Isis

The pronominal possessives hers, its, theirs, yours, and oneself have no apostrophe.
所有格代词(hers,its,theirs,yours,oneself)不加撇号。

注解

单数名词的所有格并没有严格的标准,bus'和bus's都可以。大家可以按照《风格的要素》里的建议,全部加's。

以s结尾的复数名词的所有格直接加’。具体规则可以参考《坑爹的'和's 撇号的两大用法误区(下)》