Exodus 15:16

16 Fear and dread
The two words can form a nominal hendiadys, “a dreadful fear,” though most English versions retain the two separate terms.
will fall
The form is an imperfect.
on them;
by the greatness
The adjective is in construct form and governs the noun “arm” (“arm” being the anthropomorphic expression for what God did). See GKC 428 #132.c.
of your arm they will be as still as stone
For a study of the words for fear, see N. Waldman, “A Comparative Note on Exodus 15:14–16, ” JQR 66 (1976): 189-92.

until
Clauses beginning with עַד (’ad) express a limit that is not absolute, but only relative, beyond which the action continues (GKC 446-47 #138.g).
your people pass by, O Lord,
until the people whom you have bought
The verb קָנָה (qanah) here is the verb “acquire, purchase,” and probably not the homonym “to create, make” (see Gen 4:1; Deut 32:6; and Prov 8:22).
pass by.
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