Vague Pronouns

When the reader can't determine the antecedent (the noun) to which a pronoun is referring, it interferes with the meaning of the sentence. It is important to use pronouns correctly so that the reader always knows to what or whom you are referring.
Example: When Maria and Sonja left for school, she forgot to lock the front door. (Vague)
When Maria and Sonja left for school, Maria forgot to lock the front door. (Clear)