A jussive is a third person verb. It expresses volition such as command (“let there be light”), benediction (“may he bless you”), or wish (“may the king live”). It’s form is nearly identical to the Imperfect. How can we distinguish an Imperfect (usually future) verb from a Jussive?
(1) The accent on a Jussive verb shifts one syllable toward the beginning of the word.
(2) The Jussive is at the beginning of its clause whereas an Imperfect does not come first in a clause (except a vac-conversive Imperfect).
(3) III-ה verbs with a II-י such as יהיה cut off the ה in the Jussive form, as in יהי.
(4) II-ו and II-י verbs are often spelled defectively in the Jussive form (they lose the vav or yod).