A noun can be made possessive when it could also have of a or of the preceding it.
the bag of a student = a student’s bag
the orbits of the planets = the planets’ orbits
A singular noun is usually made possessive by adding ’s to the end of the word.
The woman’s coat is red.
Most proper names are made possessive by adding ’s to the end.
Wong’s argument is compelling.
A singular noun that ends in s can be made possessive either by adding ’s to the end of the word or by adding only ’ to the end of the word.
A Christmas Carol is probably Dickens’ / Dickens’s best loved work.
The moss’ / moss’s tendency is to grow only on the north sides of trees.
Classical names ending in s as well as names ending with an s and an “iz” sound traditionally use only an apostrophe to mark possession.
Herodotus’ sense of history is still with us today.
There are many allusions to the sea in Menzies’ poetry.
A plural noun that ends in s can be made possessive either by adding only ’ to the end of the word (the preferred method), or by adding ’s to the end of the word.
All the soldiers’ / soldiers’s uniforms were torn.
A plural noun that ends in a letter other than s can be made possessive by adding ’s to the end of the word.
The men’s curling match will take place at 2:00, and the women’s will take place at 5:00.
Possessive pronouns generally do not use an apostrophe to indicate possession. This rule also applies to the possessive form of it, which is its.
Do you see that woman over there? Her dog is very friendly.
He was late for work because his car did not start this morning.
Is that your house? No, ours is the one beside it.
Virtue is its own reward.
Some possessive pronouns do, however, use ’s.
That meal would not be to everyone’s taste.
Note: it’s is not a possessive but a contraction (short form) for it is.
It’s warm in here. = It is warm in here.