Declension
For example, in English, if there is more than one boy, the word must become boys. For example, "My family has two girls and one boy, but my wife's family has two girls and three boys". The form boy is singular, meaning there is only one of the noun, and the form boys is plural, meaning there is more than one of the noun. You must know the noun's singular form if you want to find it in an English dictionary.