That vs. Who
The Grammar Guru’s pet peeve: “people that.” This rule is simple: A person is always a who, never a that. Despite its simplicity, this is one of the most often violated rules of grammar. I have seen this error in many well-respected publications. Part of the problem is that authorities are divided, and have been since the time of Chaucer.
The general rule (according to Fowler*), is that one should use who when referring to a human being, and that when referring to an inanimate object; either who or that may be used with an “animate but non-human” creature, or for a person representing a large class of like individuals (babies that cry all night).
Here is an egregious, but not unheard of, example of the wrong usage: The man that stole the money and threatened to shoot everyone that was in the bank was apprehended by the security guard that chased him down the street.
Right usage would be: The man who stole the money and threatened to shoot everyone who was in the bank was apprehended by the security guard who chased him down the street.
The following version would be better, as it would take the sentence out of passive construction and into an active voice as well as removing some lengthy clauses: After chasing him down the street, the security guard apprehended the man who stole the money and threatened to shoot everyone who was in the bank.
There is, however, one instance in which you will use that to refer to a person: when you are using that as a “demonstrative adjective” — a word denoting a particular person, place, or thing.
Example: That man is the person who robbed the bank.
*”Fowler” is the book Fowler’s Modern English Usage. If you aspire to a writing career, this book should not only be on your reference shelf, but it should also be the most worn book on that shelf. Study this book, learn it thoroughly, and you will not need The Grammar Guru because you will be a grammar guru yourself.