Pronouns are useful as substitutes for nouns, but a poorly chosen pronoun can obscure the meaning of a sentence. Common pronoun errors include:
Writers should spend time thinking about their arguments to make sure they are not superficial.
(Unclear antecedent: who or what are superficial?)
A key difference between banking crises of today and of yesterday is that they have greater global impact.
(Which crises have more impact?)
A key difference between banking crises of today and yesterday is that today's crises have greater global impact.
Pope Gregory VII forced Emperor Henry IV to wait three days in the snow at Canossa before granting him an audience. It was a symbolic act.
To what does it refer? Forcing the Emperor to wait? The waiting? The granting of the audience? The audience? The entire sentence?
Use a pronoun as subject only when its antecedent is crystal clear.
In the original state constitution, they allowed polygamy.
They (plural) refers to constitution (singular).
The original state constitution allowed polygamy.
It is often better to use a plural noun and pronoun than to use a singular noun and pronoun. Note that indefinite pronouns such as each and everyone are singular.
Each student must meet his or her advisor. (correct but awkward)
Each student must meet with their advisor. (incorrect: singular noun, plural pronoun)
Students must meet with their advisors. (correct: plural noun and pronoun)