Trivia In the sentence if they be found, why is be used after they?

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It is an example of the use of the subjunctive mood, whereby the normally present indicative form of the verb (“are”) is changed to the subjunctive form, in this case, the infinitive “be.”

Why is the subjunctive mood used?

Because it is a case of uncertainty or contrariness to fact, as represented by the use of the conjunction “if.”