Query about past tense in Samskrt

[S Krishna]

In samskrt, the ending "sma" is sometimes used to convert the
present tense into past tense i.e. "pArvati patirUpeNa zivaM
prAptumicchati sma." As is well know, "sma" cannot be used with
the prathama and the dvitIya puruSa. Is there any reason given for
this? What is the special relationship between "sma" and the third
person ? All pointers will be appreciated

Regards,
Krishna

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