باید

Inherited from Middle Persian [Book Pahlavi needed] (ʾp̄ʾyt' /⁠abāyēd⁠/, “it is necessary, fitting; must”), Manichaean Middle Persian 𐫀𐫁𐫀𐫏𐫅 (ʾbʾyd).^[1]

Originally the third-person singular present of the verb بایستن (bāyistan / bâyestan, “to be necessary”), hence etymologically "it is necessary". The original verb is now obsolete except in a few conjugations, of which باید (bāyad / bâyad, “should; must; have to”) is the most common and important.

باید • (bāyad / bâyad) (Tajik spelling бояд)

1. (defective) Expresses an obligation or confident conjecture, equivalent to English must, have to, should, ought, etc.

   باید برود (formal) ― bāyad / bâyad birawad / beravad ― he/she/they must go

   آن باید علی باشد (formal) ― ān  / ân bāyad / bâyad alī bāšad / ali bâšad ― that must be Ali

   باید رفت ― bāyad / bâyad raft ― one must go [impersonal]

   باید رفته باشد (formal) ― bāyad rafta bāšad / bâyad rafte bâšad ― he/she should have gone [but it is not clear if he/she did]; or, he/she must have gone [a conjecture about a past event]

   باید می‌رفت (formal) ― bāyad mē-raft / bâyad mi-raft ― he/she had to go [and did go]; or, he/she should have gone [but did not go]

   • c. 1010, Abū’l-Qāsim Firdawsī, Shāhnāma:

     از آغاز باید که دانی درست
     سر مایهٔ گوهران از نخست
     که یزدان ز ناچیز چیز آفرید

     az āġāz bāyad ki dānī durust
     sar māya-yi gawharān az naxust
     ki yazdān zi nāčīz čīz āfarīd

     From the start, you ought to know well
     The source of the essences from the beginning
     That God created a something from nothing.

     (Classical Persian romanization)

   • 2009, شادمهر عقیلی, “تقدیر”, (Iranian Persian):

     باید تورو پیدا کنم

     bâyad to-ro peydâ konam

     I have to find you

• The negated نباید (nabāyad / nabâyad) means “must not” as a prohibition, and not “to not have to” as a lack of an obligation. نباید بروی (nabāyad birawī / nabâyad beravi) thus means “you must not go,” not “you don’t have to go.” To express “to not have to,” use expressions such as مجبور نبودن (majbur na-budan, “to not be obliged”), لازم نیست (lâzem nist, “it’s not necessary”) or use expressions like حتما (hatman) e.g. نباید که حتما بروی (nabāyad ki hatman birawī / nabâyad ke hatman beravi, “you're not obligated to go.”)

• See بایستن (bāyistan / bâyestan).