How do you explain the rules is RELATIVE PRONOUNS (who, which, that ) ?? = REWRITE: How do you explain the rules for (the use of) the relative pronouns "who," "which," and "that"?
1. For PEOPLE: who, whoever, whosoever, whom, whomever, whom, for whom, whose (possession), that
2. For THINGS: what, whatever, that (restrictive clause), which (nonrestrictive clause), whichever, of which, in which
As you can see, "that" can be used for people or things. I prefer to use "who" over "that" for a person.
I also liked what Tina Blue wrote: http://grammartips.homestead.com/relatives.html