The declension of a word indicates an inflection concerning number, gender or case. Declensions can be applied to nouns, pronouns and adjectives alike. Each form of inflection, including which words are inflected or not, varies from language to language. Some languages, particularly the Romance languages, are highly inflected, while others, such as English, are not.
An inflection is a grammatical change made to a word to derive extra information from it. As well as the three elements of declension, inflections can also indicate mood and aspect. Certain languages allow additional inflections to reveal certainty, evidence and surprise. Inflected verbs are called conjugations, while inflections made to nouns, pronouns and adjectives are declensions. They should not be confused with prefixes and suffixes or other additions to words that change their meaning.
Number indicates how many of something there is. English has two number states: single and plural. Such plural inflections tend to follow a regular ‘+s’ pattern, turning "mug" into "mugs." There are multiple irregularities bringing in either Old English dialects such as +en, as used in ‘children,’ or Latin forms, as in the difference between ‘datum’ and ‘data.’
Other languages approach number declension differently. Hungarian has no plural declension of nouns if the exact number of nouns is known. A unique feature of Hungarian is that the plural declension, when present, adds a vowel +k to a word. The vowel rhymes with the final vowel of the word. Japanese, on the other hand, has no plurals at all except for the pronoun plural ‘tachi.’
English has largely removed the gender declension from its language. While present in some forms of Old English, it is now only found in the pronoun as in his/hers and in foreign derived words such as alumni and alumnae. Many nouns have a female version, a male version and a natural version. For example there is mare, stallion and horse or actress. This is not always the case, as thespian is a natural version to replace either actor or actress.
The application of case to nouns, adjectives and pronouns is also more limited than in other languages. Adjectives are not put into cases. Nouns are put into the possessive only, either with the addition of an apostrophe or an apostrophe +s depending on the spelling. English pronouns are inflected with three to four inflections such as ‘I, me, my, mine.’
Languages such as Latin and Polish, plus their related tongues, are far more inflected than English. Slavic languages, for example, have seven cases such as vocative, accusative and dative. English achieves much of them using articles instead. Latin boasts a large number of declensions for all adjectives, nouns and pronouns. Some of these are regular and some are not.