On this video you can see the plural of regular and irregular words.

Plurals

To make the plural form we add an -S to words:
- 1 boy / 2 boys
- car / cars
- dog / dogs

Words ending in -Y change it to -IES (but the pronunciation is regular, adding /s/):
- lady / ladies
This change doesn't occur if there is a vowel before the -Y:
- boy / boys

But if the word already ends in an /s/ sound or a similar, we add -ES (and we pronounce it "is"):
- bus / buses
- house / houses
- box / boxes
- piece / pieces
- collage / collages

Some words are irregular. Some important ones are:
- man / men
- woman / women
- child / children
- foot / feet
- tooth / teeth
- wolf / wolves
- knife / knives
- policeman / policemen
- fireman / firemen
- person / people