The pronoun sie always begins with a capital letter and is a polite form for strangers and adults in formal situations.
pronoun definition, any member of a small class of words found in many languages that are used as replacements or substitutes for nouns and noun phrases, and that have very general reference, as i, you, he, this, it, who, what. Pronouns are sometimes formally distinguished from nouns, as in english by the existence of special objective forms, as him for he or me for i, and by nonoccurrence.
In linguistics and grammar, a pronoun (abbreviated pro) is a word that substitutes for a noun or noun phrase.
PRONOUN | meaning in the Cambridge English Dictionary