In this case, we would use "who." The correct sentence is: "The person who lent me the book is a professor," "who" is used to introduce the relative clause that describes the person (subject) who lent you the book. We also can confirm by substituting "he/she" for "who" in this context. As you said, "Who" is used when referring to the subject of a sentence.
In this case, we would use "who." The correct sentence is: "The person who lent me the book is a professor," "who" is used to introduce the relative clause that describes the person (subject) who lent you the book. We also can confirm by substituting "he/she" for "who" in this context. As you said, "Who" is used when referring to the subject of a sentence.
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Who
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Who
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