大多数名词的复数，直接 +s。

phone - phones
cup - cups
pen - pens
desk - desks
apple - apples
book - books
girl - girls

结尾是ch，x，s（或s发音）的，+es

box - boxes
watch - watches
bus - buses
sandwich - sandwiches
boss - bosses
fox - foxes

以f 或 fe 结尾的，f/fe 改称v 后，+es

wolf - wolves
wife - wives
leaf - leaves
life - lives

有些名词，有特殊的复数

child - children
woman - women
man - men
mouse - mice
goose - geese

以y，o 结尾的，没有确定的规则

baby - babies
boy - boys
family - families
toy - toys
city - cities
story - stories
butterfly - butterflies
monkey - monkeys
potato - potatoes
memo - memos
tomato - tomatoes
hero - heroes
radio - radios
zoo - zoos
kangaroo - kangaroos
photo - photos
logo - logos
typo - typos
echo - echoes

如果非要找出规律的话，

• 辅音+y，则ies；其他+s。
• 除了“potatoes, tomatoes, echoes, heroes, vetoes, embargoes, torpedoes”以外，其他以o结尾的名词，+s。

参考文献

http://www.talkenglish.com/grammar/singular-plural-nouns.aspx