You can form the possessive case of a singular noun that does not end in "s" by adding an apostrophe and "s," as in the following sentences:

The red suitcase is Cassandra's.

The only luggage that was lost was the prime minister's.

The exhausted recruits were woken before dawn by the drill sergeant's screams.

The miner's face was covered in coal dust.

You can form the possessive case of a singular noun that ends in "s" by adding an apostrophe alone or by adding an apostrophe and "s," as in the following examples:

The bus's seats are very uncomfortable.

The bus' seats are very uncomfortable.

The film crew accidentally crushed the platypus's eggs.

The film crew accidentally crushed the platypus' eggs.

Felicia Hemans's poetry was once more popular than Lord Byron's.

Felicia Hemans' poetry was once more popular than Lord Byron's.

You can form the possessive case of a plural noun that does not end in "s" by adding an apostrophe and a "s," as in the following examples:

The children's mittens were scattered on the floor of the porch.

The sheep's pen was mucked out every day.

Since we have a complex appeal process, a jury's verdict is not always final.

The men's hockey team will be playing as soon as the women's team is finished.

The hunter followed the moose's trail all morning but lost it in the afternoon.

You can form the possessive case of a plural noun that does end in "s" by adding an apostrophe:

The concert was interrupted by the dogs' barking, the ducks' quacking, and the babies' squalling.

The janitors' room is downstairs and to the left.

My uncle spent many hours trying to locate the squirrels' nest.

The archivist quickly finished repairing the diaries' bindings.

Religion is usually the subject of the roommates' many late night debates.