Source: Hart, pp. 178-179; Whitney, §1026ff

The desiderative is a secondary form that indicates a desire to perform the action of the verb. Thus, from कृ there is चिकीर्षति meaning “he desires to do;” from जीव्, there is जिजीविषामि meaning “I want to live.”

The desiderative stem is formed by reduplicating the root and adding to it –स् or –इष् after which it is treated like a normal thematic verb. That is, it takes the thematic present, imperfect, optative, and imperative endings; forms present participles; makes causative, passive, and future stems, and has a periphrastic perfect. (It cannot form a simple perfect.) All with the thematic endings.

While the rules for reduplication for the desiderative are complex (see Whitney, §1026ff), in general the consonant of the reduplicated syllable is determined by the usual rules, while the vowel is –इ– if the root has an –आ, –ई, –ॠ and –उ– if the root has an –ऊ. Some common desiderative stems such as आप् and दा have abbreviated reduplication.

Normal Forms of the Desiderative

Thus, from या (“go”) there is यियासति; from नी, निनीषति; from भू, there is बुभूषति, but from आप् there is ईप्सति, and from दा, दीत्सति.

Examples of other tenses of the desiderative are:

passive: ईप्सयते (“It is desired to be obtained.”)
causative: ईप्सयति (“He causes to desire to obtain.”)
future, simple: ईप्सिष्यति (“She will desire to obtain.”)
future, periphrastic: ईप्सितास्मि
only forms the perfect, periphrastic: ईप्सां चकार
past passive participle: ईप्सित (always the –इत form)
past active participle: ईप्सितवन्त्
present active participle: ईप्सन्त्
present middle participle: ईप्समान
future active participle: ईप्सिष्यन्त्
future middle participle: ईप्सिष्यमान
only forms the –इस् aorist: ऐप्सीत् (“he desired to get”)

Unusual Forms of the Desiderative

While all the forms above are regular permutations of thematic verbs, there are two forms of the desiderative that are different from regular thematic verbs. One quite common form is to create a present participle by adding –उ to the desiderative stem. This is declined like शत्रु m., धेनु f., and मधु n. Thus, चिकीर्षुः means “the man/woman desiring to do.” This form is often used and should be remembered.

The desiderative also forms an action noun by adding –आ to the desiderative stem. These nouns are feminine and declined like सेना. For example from पा (“to drink”), we get पिपासा (“the desire to drink”) or “thirst.”