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In grammar, '''inflection''' is the modification of a word to express different grammatical categories such as [[tense]], [[mood]], [[voice]], [[aspect]], [[person]], [[number]], [[gender]] and [[case]]. Conjugation is the inflection of verbs; declension is the inflection of nouns, adjectives and pronouns.

== Inflectional Paradigms and Inflectional Rules ==

In the UNL<sup>arium</sup> framework, inflection is indicated by a set of transformations carried over the [[lemma]]. This set of transformations can be represented by:
*'''inflectional paradigms''', in case of regular behaviour (i.e., a set of transformations that is followed by several different words)
*'''inflectional rules''', in case of irregular behaviour (i.e., a set of transformation that is followed by very few words); or
*'''inflectional paradigms''' and '''inflectional rules''', in case of quasi-regular behaviour (i.e., when the word is mainly regular but has some inflectional particularities).

For instance, the plural of English nouns is considerably regular and can be treated, in most cases, by the following '''inflectional paradigms''':

{| border="1" cellpadding="5"
| Paradigm
| Rule
| Description
| Example
|-
| 1
| PLR:=0>"s";
| Add "s" to the end of the word
| boy > boys
|- 
| 2
| PLR:="y">"ies";
| Replace "y" by "ies" at the end of the word
| city > cities
|-
| 3
| PLR:=0>"es";
| Add "es" to the end of the word
| kiss > kisses
|-
| 4
| PLR:="f">"ves";
| Replace "f" by "ves" at the end of the word
| woolf > woolves
|}

However, there are several special cases that, being very limited, should be treated by '''inflectional rules''' instead of inflectional paradigms:

{| border="1" cellpadding="5"
| Rule
| Description
| Case
|-
| PLR:="men";
| Replace the whole word by "men"
| man > men
|- 
| PLR:="mice";
| Replace the whole word by "mice"
| mouse > mice
|-
| PLR:="feet";
| Replace the whole word by "feet"
| foot > feet
|-
| PLR:="children";
| Replace the whole word by "children"
| child > children
|-
| ...
| ...
| ...
|}

The difference between inflectional paradigms and inflectional rules is mainly a question of frequency. If a rule is applicable to several different words, it should be defined as a general inflectional paradigm; if it is applicable to a single word or to a very limited number of cases, it should be defined as an inflectional rule inside the very entry.

== Syntax ==
Inflectional paradigms and inflectional rules are expressed by [[A-rule]]s, a special formalism for introducing prefixes, infixes and suffixes to the lemma.