Inflection

In linguistic morphology, inflection is a process of word formation in which a word is modified to express different grammatical categories such as tense, case, voice, aspect, person, number, gender, mood, animacy, and definiteness. The inflection of verbs is called conjugation, while the inflection of nouns, adjectives, adverbs, etc. can be called declension.
An inflection expresses grammatical categories with affixation, apophony, or other modifications. For example, the Latin verb ducam, meaning "I will lead", includes the suffix -am, expressing person, number, and tense-mood. The use of this suffix is an inflection. In contrast, in the English clause "I will lead", the word lead is not inflected for any of person, number, or tense; it is simply the bare form of a verb. The inflected form of a word often contains both one or more free morphemes, and one or more bound morphemes. For example, the English word cars is a noun that is inflected for number, specifically to express the plural; the content morpheme car is unbound because it could stand alone as a word, while the suffix -s is bound because it cannot stand alone as a word. These two morphemes together form the inflected word cars.
Words that are never subject to inflection are said to be invariant; for example, the English verb must is an invariant item: it never takes a suffix or changes form to signify a different grammatical category. Its categories can be determined only from its context. Languages that seldom make use of inflection, such as English, are said to be analytic. Analytic languages that do not make use of derivational morphemes, such as Standard Chinese, are said to be isolating.
Requiring the forms or inflections of more than one word in a sentence to be compatible with each other according to the rules of the language is known as concord or agreement. For example, in "the man jumps", "man" is a singular noun, so "jump" is constrained in the present tense to use the third person singular suffix "s".
Languages that have some degree of inflection are synthetic languages. They can be highly inflected, moderately inflected, weakly inflected, but not uninflected. Languages that are so inflected that a sentence can consist of a single highly inflected word are called polysynthetic languages. Languages in which each inflection conveys only a single grammatical category, such as Finnish, are known as agglutinative languages, while languages in which a single inflection can convey multiple grammatical roles are called fusional.

Examples in English

In English most nouns are inflected for number with the inflectional plural affix -s, and most English verbs are inflected for tense with the inflectional past tense affix -ed. English also inflects verbs by affixation to mark the third person singular in the present tense, and the present participle. English short adjectives are inflected to mark comparative and superlative forms.
There are eight regular inflectional affixes in the English language.

Affix	Grammatical category	Mark	Part of speech
-s	Number	plural	nouns
-'s/'/s	Case	genitive	nouns and noun phrases, pronouns
-ing	Aspect	progressive	gerunds or participles
-en/-ed	Aspect	perfect	verbs
-ed/-t	Tense	past	verbs
-s	Person, number, aspect, tense	3rd person singular present indicative	verbs
-er	Degree of comparison	comparative	adjectives and adverbs
-est	Degree of comparison	superlative	adjectives and adverbs

Despite the march toward regularization, modern English retains traces of its ancestry, with a minority of its words still using inflection by ablaut and umlaut, as well as long-short vowel alternation. For example:

Write, wrote, written
Sing, sang, sung
Foot, feet
Mouse, mice
Child, children

For details, see English plural, English verbs, and English irregular verbs.

Regular and irregular inflection

When a given word class is subject to inflection in a particular language, there are generally one or more standard patterns of inflection that words in that class may follow. Words which follow such a standard pattern are said to be regular; those that inflect differently are called irregular.
For instance, many languages that feature verb inflection have both regular verbs and irregular verbs. In English, regular verbs form their past tense and past participle with the ending -d. Therefore, verbs like play, arrive and enter are regular, while verbs like sing, keep and go are irregular. Irregular verbs often preserve patterns that were regular in past forms of the language, but which have now become anomalous; in rare cases, there are regular verbs that were irregular in past forms of the language.
Other types of irregular inflected form include irregular plural nouns, such as the English mice, children and women and the French yeux ; and irregular comparative and superlative forms of adjectives or adverbs, such as the English better and best.
Irregularities can have four basic causes:

euphony: Regular inflection would result in forms that sound esthetically unpleasing or are difficult to pronounce.
principal parts: These are generally considered to have been formed independently of one another, so the student must memorize them when learning a new word. Example: Latin dīcō, dīcere, dīxī, dictum → Spanish digo, decir, dije, dicho.
strong vs. weak inflection: In some cases, two inflection systems exist, conventionally classified as "strong" and "weak." For instance, English and German have weak verbs that form the past tense and past participle by adding an ending and strong verbs that change vowel, and in some cases form the past participle by adding -en. Ancient Greek verbs are likewise said to have had a first aorist and a second aorist.
suppletion: The "irregular" form was originally derived from a different root. The comparative and superlative forms of good in many languages display this phenomenon.

For more details on some of the considerations that apply to regularly and irregularly inflected forms, see the article on regular and irregular verbs.

Declension and conjugation

Two traditional grammatical terms refer to inflections of specific word classes:

Inflecting a noun, pronoun, adjective, adverb, article or determiner is known as declining it. The forms may express number, case, gender or degree of comparison.
Inflecting a verb is called conjugating it. The forms may express tense, mood, voice, aspect, person, or number.

An organized list of the inflected forms of a given lexeme or root word is called its declension if it is a noun, or its conjugation if it is a verb.
Below is the declension of the English pronoun I, which is inflected for case and number.

	singular	plural
nominative	I	we
oblique	me	us
possessive determiner	my	our
possessive pronoun	mine	ours
reflexive	myself	ourselves

The pronoun who is also inflected according to case. Its declension is defective, in the sense that it lacks a reflexive form.

	singular and plural
nominative	who
oblique	whom, who
possessive	whose
reflexive	–

The following table shows the conjugation of the verb to arrive in the indicative mood: suffixes inflect it for person, number, and tense:

Tense	I	you	he, she, it	we	you	they
Present	arrive	arrive	arrives	arrive	arrive	arrive
Past	arrived	arrived	arrived	arrived	arrived	arrived

The non-finite forms arrive, arrived and arriving, although not inflected for person or number, can also be regarded as part of the conjugation of the verb to arrive. Compound verb forms, such as I have arrived, I had arrived, or I will arrive, can be included also in the conjugation of the verb for didactic purposes, but they are not overt inflections of arrive. The formula for deriving the covert form, in which the relevant inflections do not occur in the main verb, is

Inflectional paradigm

An inflectional paradigm refers to a pattern, where a class of words follow the same pattern. Nominal inflectional paradigms are called declensions, and verbal inflectional paradigms are termed conjugations. For instance, there are five types of Latin declension. Words that belong to the first declension usually end in -a and are usually feminine, these words share a common inflectional framework. Meanwhile words that belong to the second declension end in -us, -um, or -er, and are usually masculine or neuter. In Old English, nouns are divided into two major categories of declension, the strong and weak ones, as shown below:
The terms "strong declension" and "weak declension" are primarily relevant to well-known dependent-marking languages. In dependent-marking languages, nouns in adpositional phrases can carry inflectional morphemes.
In head-marking languages, the adpositions can carry the inflection in adpositional phrases. This means that these languages will have inflected adpositions. In Western Apache, the postposition -ká’ 'on' is inflected for person and number with prefixes:
Traditional grammars have specific terms for inflections of nouns and verbs but not for those of adpositions.