بایستن

بایستن • (bâyestan), present stem بای‎ (bây)

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”)