The sentence that uses an auxiliary verb to indicate certainty is "The magi brought valuable gifts, but that was not among them. This dark assertion will be illuminated later on.
Auxiliary verbs is the name given to verbs that help to shape the mood and voices of other verbs, otherwise known as "helping verbs".
Thus, the sentences that are indicative of Auxiliary verbs are:
"The magi brought valuable gifts, but that was not among them. This dark assertion will be illuminated later on."
Learn more about Auxiliary Verbs at:
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