“Form the possessive singular of nouns by adding ‘s.
Follow this rule whatever the final consonant. Thus write,
The witch’s malice
Exceptions are the possessives of ancient proper names ending in –es and ‘is, the possessive Jesus’, and such forms as for conscience’ sake, for righteousness sake. But such forms as Moses’ Laws, Isis’ temple are commonly replaced by
The laws of Moses
The temple of Isis
The pronominal possessives hers, its, theirs, yours and ours have no apostrophe. Indefinite pronouns, however, use the apostrophe to show possession.
Somebody else’s umbrella
A common error is to write it’s for its, or vice versa. The second is possessive.
It’s a wise dog that scratches its own fleas.”
Excerpted from: Strunk, William Jr., and E.B. White. The Elements of Style, Fourth Edition. New York: Longman, 2000.