In linguistics, a desiderative (abbreviated DESI or DES) form is one that has the meaning of "wanting to X". Desiderative forms are often verbs, derived from a more basic verb through a process of morphological derivation.
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Sanskrit
In Sanskrit, the desiderative is formed through the suffixing of /sa/ and the prefixing of a reduplicative syllable, consisting of the first consonant of the root (sometimes modified) and a vowel, usually /i/ but /u/ if the root has an /u/ in it. Changes to the root vowel sometimes happen, as well.
For example:
Meadow Mari
In Meadow Mari, the desiderative mood is marked by the suffix -не -ne.
Japanese
In Japanese, the desiderative takes two main forms: -tai (-たい) and -tagaru (-たがる). Both forms conjugate for tense and positivity, but in different ways: with the -tai ending, the verb becomes an -i adjective, or a conjugable adjective, while the ending -tagaru creates a godan/yodan verb. Though there are other, compound forms to demonstrate wanting, these two alone are demonstrated because they are inflections of the main verb. These two forms are plain/informal in nature, and can be elevated to the normal-polite and other levels through normal methods.
-tai is an absolute statement of desire, whereas -tagaru indicates the appearance of desire. Generally, one does not say things such as 太郎さんが食べたい Tarō wants to eat because one cannot read Tarō's thoughts; instead, one says 太郎さんが食べたがる it appears that Tarō wants to eat.
Proto-Indo-European
Proto-Indo-European likely had a desiderative. In some daughter languages like Indo-Iranian, Balto-Slavic and possibly Celtic, it acquired the meaning of a future tense.