بایستن

Inherited from Middle Persian [Book Pahlavi needed] (ʾp̄ʾdstn' /⁠abāyistan⁠/, “be neccassary, fiting; desire, want”),^[1] from Proto-Iranian *upa- + *Hai, from Proto-Indo-Iranian *úpa + *Hay-, from Proto-Indo-European *upó (“under, below”) + *h₁ey- (“to go”).

بایستن • (bāyistan / bâyestan) (present stem بای (bāy / bây), Tajik spelling боистан)

1. (defective, auxiliary) to be necessary; must, ought

Only the following conjugations now exist:

• باید (bâyad, etymologically third-person singular present, literally “it is necessary”), extremely commonly used as the equivalent to English constructions with "must", "should", "ought to", "have to", etc.
• می‌باید (mi-bâyad), synonymous to باید (bâyad) but less common
• بایست (-bâyest, etymologically third-person singular past), usually largely synonymous to باید (bâyad) despite the use of the past stem, but less common and somewhat more polite; more pedantic usage may have this mean "should have", "must have", etc., preserving the past tense
• می‌بایست (mi-bâyest), synonymous to بایست (bâyest)
• بایستی (bâyesti, etymologically optative third-person singular past) and می‌بایستی (mi-bâyesti, etymologically optative third-person singular imperfect), both rather archaic and used to express any past necessity without regard for grammatical person

• بایستی (bâyesti, “necessity”)

• Cheung, Johnny (2007), Etymological Dictionary of the Iranian Verb (Leiden Indo-European Etymological Dictionary Series; 2), Leiden, Boston: Brill, →ISBN, pages 154-157