Plurales irregulares en inglés:

The spelling of some nouns are completely changed when forming the plural. Unfortunately there isn’t a rule as such, you simply have to learn them.
Here are some common examples:

1 MAN is – 2 MEN are
1 WOMAN is – 2 WOMEN are
1 CHILD is – 2 CHILDREN are
1 FOOT is – 2 FEET are
1 PERSON is- 2 PEOPLE are
1 TOOTH is – 2 TEETH are
1 MOUSE is – 2 MICE are

Names of professions with the word MEN or WOMEN such as

1 POLICEMAN is – 2 POLICEMEN are
1 POLICEWOMAN is – 2 POLICEWOMAN are
1 POSTMAN is – 2 POSTMEN are
1 POSTWOMAN is- 2 POSTWOMEN are
These plurals never need a S on the end. You should never say Peoples, Teeths or Womens when referring to plurals.
Some irregular plurals have the same form as the singular.
1 SHEEP is – 2 SHEEP are
1 FISH is – 2 FISH are
1 DEER is – 2 DEER are

All this might seem complicated at first, but once you get used to it you will be ok.