TYPICAL ERROR Isabella told Sarah that she had passed the exam.
CORRECTION Isabella told Sarah that she, Isabella, had passed the exam.
EXPLANATION In the typical error, it is not clear whether the word she refers to Isabella or to Sarah; we can’t tell whether it was Isabella or Sarah who had passed the exam. (The pronoun’s referent, the word to which it refers, is indefinite, so the sentence is ambiguous.)
In everyday speech, we sometimes hear phrases where the referent for a pronoun is not merely indefinite; it is non-existent. The most common example is They say, as in:
They say that the world is heating up.
Such statements should be avoided in writing because they has no referent; we do not know who they are. A better sentence would be:
Most scientists accept that the world is heating up.
The following sentences are ambiguous. Replace each pronoun with the noun that you think is its intended referent (i.e. the noun that you think it refers to). (There may be more than one valid solution.)