When do you add 's' at the end of a word?

There are several reasons for adding -s to a Word.

1

To make a plural noun:

one cat; two cats

2

To make the third person singular of the simple presenttense of a verb:

I work; she works, he works, it works

3

With a preceding apostrophe, to denote ownership or possession:

Elizabeth's crown; the cat's basket

4

With a following apostrophe, to denote plural possession:

the cats' food [= the food for the cats]