Three types of words have an s ending:

Plural Nouns

For most nouns, you add an s when you want to make a plural.

• How many jars of pickles did they buy?
• Girls, please ride your bicycles to the post office and mail these letters.

Of course, some nouns have plurals with no s.

• In New Zealand, there are more sheep than people.
• The wildlife in the wetlands includes mice, geese and moose.

Third Person Singular Verbs

When you can substitute the pronouns he, she or it for a noun in your sentence, that noun is third person singular. Most verbs following those nouns end in s.

• We run once a week, but he runs every day.
• She buys a new car every other year. The latest one looks quite sporty.

Nouns Showing Possession

To show that one thing possesses another, add an apostrophe and an s to the first noun. You can recognize this situation when you see two nouns together and the second belongs to the first. The sentence won't make sense unless you add the apostrophe to show possession.

• The girl's mother was upset about today's weather.
• Are they going to change the school's name?
• Study Darwin's theory for tomorrow's discussion.

If a noun is made plural by adding an s, put the apostrophe after the s.

• The homeowners' association meets on the first Tuesday of every month.
• It was formerly a boys' club, but now they are accepting girls.