Fusional languages or inflected languages are a type of synthetic language, distinguished from agglutinative languages by their tendency to use a single inflectional morpheme to denote multiple grammatical, syntactic, or semantic features.
For example, the Spanish verb comer ("to eat") has the first-person singular preterite tense form comí ("I ate"); the single suffix -í represents both the features of first-person singular agreement and preterite tense, instead of having a separate affix for each feature.
Another illustration of fusionality is the Latin word bonus ("good"). The ending -us denotes masculine gender, nominative case, and singular number. Changing any one of these features requires replacing the suffix -us with a different one. In the form bonum, the ending -um denotes masculine accusative singular, neuter accusative singular, or neuter nominative singular.
Examples of fusional Indo-European languages are: all Balto-Slavic languages which includes Polish, Russian, and Ukrainian; Sanskrit, Pashto, modern Indo-Aryan languages such as Hindustani, Kashmiri, and Punjabi; Greek (classical and modern), Latin, Italian, French, Spanish, Portuguese and Romanian; Irish, German, Faroese, Icelandic, Albanian.
Northeast Caucasian languages are weakly fusional.
Fusional languages generally tend to lose their inflection over the centuries – some languages much more quickly than others.
While Proto-Indo-European was fusional, some of its descendants have shifted to a more analytic structure, such as Modern English, Danish and Afrikaans, or agglutinative, such as Persian and Armenian.
Some languages shift over time from agglutinative to fusional.
For example, while most Uralic languages are predominantly agglutinative, Estonian is markedly evolving in the direction of a fusional language. On the other hand, Finnish, its close relative, exhibits fewer fusional traits, thereby keeping closer to the mainstream Uralic type. However, Sámi languages, also part of the Uralic family, have gained more fusionality than Finnish and Estonian.
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One feature of many fusional languages is their systems of declensions. Here nouns and adjectives have a suffix attached to them to specify grammatical case (their uses in the clause), number, and grammatical gender; pronouns may alter their forms entirely to encode this information.
Within a fusional language, there are usually more than one declension; Latin and Greek have five, and Slavic languages have anywhere between three and seven. German has multiple based on whatever vowel or consonant the word ends with and can often be unpredictable.
However, many descendants of fusional languages tend to lose their case marking. In most Romance and Germanic languages, including modern English (with the notable exceptions of German and Icelandic), encoding for case is merely vestigial; this is because it no longer encompasses nouns and adjectives, but only pronouns.
Compare the Italian egli (masculine singular nominative), gli (masculine singular dative, or indirect object), lo (masculine singular accusative) and lui (also masculine singular accusative, but emphatic and indirect case to be used with prepositions), corresponding to the single vestigial pair he, him in English.
Conjugation is the alteration of the form of a verb to encode information about some or all of grammatical mood, voice, tense, aspect, person, grammatical gender, and number. In a fusional language, two or more of these pieces of information may be conveyed in a single morpheme, typically a suffix.
For example, in French, the verbal suffix depends on the mood, tense, and aspect of the verb, as well as on the person and number (but not the gender) of its subject. This gives rise to typically forty-five different single-word forms of the verb, each conveying some or all of the following:
To change any one of these pieces of information without changing the others requires using a different suffix, the key characteristic of fusionality.
English has two examples of conjugational fusion. The verbal suffix -s indicates a combination of present tense with both third person and singularity of the associated subject, and the verbal suffix -ed, used in a verb with no auxiliary verb, conveys both non-progressive aspect and past tense.