is this sentence essential or nonessential? This theory, which may at first seem like a old wives tale, actually has some basis in fact.

Answer

nonessential

Q: What is an essential sentence? A: An essential sentence provides necessary information to the meaning of the sentence. Without it, the sentence would not make sense or be complete.
Q: What is a nonessential sentence? A: A nonessential sentence adds additional information that is not necessary for the sentence to make sense or be complete.
Q: Is the sentence 'This theory, which may at first seem like an old wives tale, actually has some basis in fact' necessary to the meaning of the sentence? A: No, the sentence adds additional information, making it a nonessential sentence.