The rule in here is to form the possessive singular of nouns with 's. 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.
Nouns refer name a person, place, thing, event, or idea. In the English language, nouns commonly function as the subject of the sentence. Nouns seem to be the simplest among the other parts of speech, yet it sometimes confuses a reader.