They is Plural
Writers have a problem these days getting subject-pronoun agreement, because ever since the women's movement in the seventies, it has been considered bad form to use "he" as the basic pronoun. In the bad old days, one would write:
A person must be careful to do whatever he thinks is right.
But, nowadays, that would be considered sexist language, since the general word "person" might refer to either a man or a woman. So, one might write:
A person must be careful to do whatever he/she thinks is right.
This is correct, because it is non-sexist, but it is mighty awkward. The easiest way to deal with the problem is to make the subject plural:
People must be careful to do whatever they think is right.
But that is not always convenient. Another solution sometimes used is to switch between "he" and "she" throughout the paper, but that is awkward too.
Writers often try to solve this problem by using "they" as the pronoun for a singular subject, but that is NOT CORRECT. An example of this common error is:
A person must be careful to do whatever they think is right.
NO. NO. NO. They is plural; person is singular--not a match!
Frankly, there is no easy solution; you will have to work each writing out for yourself so that the subjects and their pronouns agree in number without being sexist. I think that eventually "they" will be declared either singular or plural, but that won't happen in my lifetime, and probably not in yours either, so learn to avoid this numerically incorrect solution.
