Here's why:
* Subject Complement: A subject complement is a word or phrase that renames or describes the subject of a sentence. It follows a linking verb (like "is," "are," "was," "were," "seems," "becomes," etc.).
* Personal Pronoun: A personal pronoun is a word that replaces a noun (e.g., I, you, he, she, it, we, they).
Examples:
* Correct: She is my sister. ("Sister" is the subject complement, renaming the subject "She.")
* Incorrect: She is her. (This is grammatically incorrect because "her" is a personal pronoun and cannot act as a subject complement.)
To make the sentence grammatically correct, you could say:
* She is my sister.
* She is the one who came to visit.
Key Takeaway: Subject complements must describe or rename the subject. Personal pronouns are already replacements for nouns, so they can't further rename or describe the subject.