What is the subject? Is "whom" the subject? Her sons, both of whom work abroad, will come back home this summer.
Question is nice.
Both of whom it is a subordinate clause or dependent clause of-- Her sons will come back home this morning--it is independent a clause.
But in the subordinate clause both is the subject but whom is an object of the preposition -OF.
Whom is an object pronoun. But sons refers to whom to by BOTH.
Hope it works.
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