Declension

In linguistics, declension is the changing of the form of a word. This shows its function in the sentence, by way of some inflection. The inflectional change of verbs is called conjugation.
Declensions may apply to nouns, pronouns, adjectives, adverbs, and articles to indicate number (e.g. singular, dual, plural), case (e.g. nominative case, accusative case, genitive case, dative case), gender (e.g. masculine, neuter, feminine), and a number of other grammatical categories.
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.
Also in English, a noun can change if it owns something. For example, if a toy belongs to one boy, the toy is the "boy's" toy, but if it belongs to two or more boys, it becomes the "boys"' toy. Both numbers of the noun have this word case, which is called the genitive case. Word case is when a word form changes depending on what part of the sentence it is.