Noun declension refers to a process by which a noun changes class in usage, often through a change in the actual form of the word itself. There are different possible cases for different words, and in English these different cases usually indicate the way in which a word, such as a noun, is used in a sentence. The nominative case, for example, is the form of a noun that is used as the subject of a sentence. On the other hand, the accusative case is used for a word as the direct object in a sentence. Noun declension indicates that nouns decline or inflect through different cases, sometimes due to the use of different forms.
One of the complicated aspects of noun declension for native English speakers is that many nouns in English do not demonstrate declension through any difference in form. For example, the noun “book” does not change form between its used in the nominative case, “This book is good,” and its use in the accusative case, “I like this book.” In other languages, however, noun declension is indicated by a change in form for many words, and this is often seen in Latin and languages that developed from it such as Spanish and Italian. Even German, from which many words and grammatical structures in English were derived, uses more changes in words to indicate declension or inflection.
Noun declension still occurs in English; it simply does not happen through the use of different word forms in most cases. Plurality is often indicated through a shift, usually by the addition of the suffix “-s” or “-es,” though some words shift more dramatically, such as “mouse” and “mice.” Noun declension is also used in some languages to indicate gender, usually through masculine, feminine, and neuter words. In English, this is only found for a handful of words, usually those associated with roles people can take. This is often adjusted from a base word through a suffix that indicates a feminine role, such as the shift from “actor” to “actress.”
Some pronouns in English use declension to indicate different cases and even possessive forms. The difference between “him” and “he,” for example, is similar to how noun declension is used in some other languages. “He” is used as the nominative case for a singular, masculine pronoun; while the accusative case of a singular, masculine pronoun is “him.” “She” and “her” are used in similar ways, and the masculine pronoun also demonstrates possession through a form change to “his.”