Adding -s
 
We add -s to words for two reasons:
to make plural nouns (boy > boys) 
to form the 3rd person singular of the present simple tense (I work > you work > he works) 
Spelling Rule
Just add -s to the end of the word, for example:
dog > dogs 
play > plays 
demand > demands 
Exceptions
If the word ends in:
 do this:
 and add:
 For example:
 
-ch
-s
-sh
-x
-z
 nothing
 -es
 church > churches
mass > masses
brush > brushes
fax > faxes
box > boxes
chintz > chintzes
 
-f
-fe
 remove the -f or -fe 
 -ves
 wife > wives
calf > calves
except:
beliefs, chiefs, dwarfs, griefs, gulfs, proofs, roofs
 
consonant + -y
 remove the -y
 -ies
 spy > spies
baby > babies
 
Note: words that end in -o normally just add s, except:
buffalo > buffaloes
cargoes (or cargos)
domino > dominoes
echo > echoes
go > goes
grotto > grottoes
halo > haloes
hero > heroes
mango > mangoes
mosquito > mosquitoes
motto > mottoes (or mottos)
potato > potatoes
tomato > tomatoes
tornado > tornadoes
torpedo > torpedoes
veto > vetoes
volcano > volcanoes