When writing a sentence, using the same word more than once can get repetitive.
Example: When a woman is sick, the woman should call a doctor.
Rather than repeating woman twice, we could use a pronoun.
Example: When a woman is sick, she should call a doctor.
Since woman is singular (there is only one), use a singular pronoun (she) to replace it.
What would happen if we made the subject of our sentence plural?
Example: When individuals are sick, individuals should call a doctor.
Now, since individuals is plural, we should choose a plural pronoun to replace it:
Example: When individuals are sick, they should call a doctor.
Finally, if there are multiple single subjects, we must use a plural pronoun:
Example: When a supervisor and an employee are sick, they should call a doctor.
While pronouns are useful to help writers avoid repetition, pronouns should be used sparingly to keep the meaning of the sentence clear. Take a look at this sentence:
Example: When Jeff and Brian joined the team members, they were scared.
The pronoun here (they) is unclear--to whom does it refer? Was the team scared? Were Jeff and Brian scared? Because the pronoun they is ambiguous, choosing a noun rather than a pronoun will help with clarity.
Example:When Jeff and Brian joined the team members, the team members were scared.