I have been told that the sentence: "Another element is Mrs. Hale’s act of pulling the stitches from Mrs. Wright’s quilt." is a fragment. I thought "Another element" is the subject of the sentence since "another" is a pronoun and "element" a noun. The "is" in the sentence is an auxiliary verb and ending a sentence with a prepositional phrase ("of pulling the stitches from Mrs. Wright's quilt") is not an error. What am I not seeing? Thanks so much!
Your sentence is a sentence, not a fragment (assuming that 'fragment' means incomplete sentence). I'd call 'is' a main verb rather than an auxiliary verb, so the sentence is of the form A is B. The sentence would still be alright grammatically, though more puzzling to the reader, if it ended on the word 'act'.