That vs. Who, Whose and Whom
“I hear people say “that” for other people all the time. “Who” is for people; “that” is for everything else.
The hack: Who and whom are for humans. Think human — who-man.
Examples:
The people who work here are nice.
John is the guy to whom my friend is married. (Not: John is the guy that my friend is married to.)
The book that she’s reading looks good.”