In linguistics, a desiderative (abbreviated DESI or DES) form is one that has the meaning of "wanting to X".
For example: In Meadow Mari, the desiderative mood is marked by the suffix -не -ne.
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 (-tai + -garu suffix) creates a godan/yodan verb.
These two forms are plain/informal in nature, and can be elevated to the normal-polite and other levels through normal methods.
In some daughter languages like Albanian, Indo-Iranian, Balto-Slavic and possibly Celtic, it acquired the meaning of a future tense.