It's All Relative.

Hello Readers,
  This month I explain how to use a relative pronoun correctly.  What is a relative pronoun?
A relative pronoun connects, or "relates", one clause to another instead of using two separate sentences.
Example 1: 
      Bobby Fischer is the great chess player.  He beat Boris Spassky in 1972.  [Two sentences]
      Bobby Fisher is the great chess player who beat Boris Spassky in 1972.  [One sentence with a relative pronoun linking the two.]
Example 2: 
      Chess is a game.  This game is very challenging.  [Two sentences]
      Chess is a game that is very challenging.  [One sentence with a relative pronoun.]

Did you notice that there are two possible relative pronouns?  How do we decide which one to use?  Simple.  The relative pronoun who references a person.  If you reread Example 1, you will see that the word replaced by who was "He", referring to Bobby Fischer. 
The relative pronoun that references a thing.  In Example 2, you will see that the words replaced by that  were "the game", which is a thing.   Therefore, the rule is:
Use the relative pronoun who when referring to or replacing a reference to people and
use the relative pronoun that when referring to or replacing a thing.

Here are some examples of how the wrong relative pronoun is used.

In the 2006 film "Basic Instinct 2", psychiatrist Dr. David Glass defends himself.  "I was treating a patient that ended up killing his pregnant girl friend."  WRONG.  Since the reference is "a patient", the pronoun should be who.  CORRECT:  I was treating a patient who ended up killing his pregnant girl friend.

On "America's Got Talent" Episode 14 from 2012, Sharon Osbourne says about young dance contestant Lil Star, "We are talking about a girl that is 6 years old."  WRONG.  Again, the reference is a person and the relative pronoun should be who.  CORRECT:  We're talking about a girl who is 6 years old.

In the episode "A Load of Bull" on tv's "Necessary Roughness", therapist Dr. Dani Santino explains to a patient:  "You have this incredible wife that wants to take the journey with you."  WRONG.  In most cases, a wife is a person.  CORRECT:  You have this incredible wife who wants to take the journey with you.

Fictional crime writer Richard Castle, of the titular tv show "Castle", is bantering with his now love interest NYPD Detective Kate Beckett:  "It was you that was trying to sleep with me!"  WRONG.  We all know that he is referring to a person, and he is a writer!  CORRECT:  It was you who was trying to sleep with me.

Heard on the news in June, 2012:  "Next up, Derek Luke, the man that played Antwone Fisher."
WRONG.  By now, we know that it should be "... the man who played Antwone Fisher."

Heard on a commercial for Scan Health Plan:  "...one of the big differentiators of Scan is the level of trust and competence you have with a company who is started by seniors..."  WRONG.  The reference is a company which is a thing.  The CORRECT pronoun should be that.   "... a company that is started by seniors."

Stephen King's novel From a Buick 8 is about a mysterious car that shows up at Pennsylvania State Troop D's headquarters.  [Note my use of the relative pronoun that.]  The troopers' canine mascot
"started barking.  Not his deep-throated bark, the one he usually saved for the deer who scouted out the back field or the raccoons that dared come sniffing around..."  The first relative pronoun is WRONG because deer are not people.  It should say, "... the deer that scouted..."  The second relative pronoun is CORRECT because, like deer, raccoons are not people.

Now some examples of the correct usage of these pronouns.

On the tv show "Vegas", episode "Exposure", Sheriff Lamb interviews a man about a murder on the nearby Air Force Base.  The man explains, "I'm a civilian who works on the base."  YES!  He is indeed a person.

The 2012 tv movie "Blue Lagoon:  The Awakening" tells the story of Dean and Emma who are stranded on an island.  [Note my use of the relative pronoun who.]  He reassures her by saying, "You've got people out there who love you...people looking for you." Hurray!  The very reference was people, so the relative pronoun was correct.

In the episode "Jane's Getting Married" of tv's "Drop Dead Diva", attorney Jane Bingum's colleague wants her off a case he is working on.  He says, "Mark needs lawyers who believe in him."
Contrary to popular belief, lawyers are also people, and here the relative pronoun is correct.

Seen on an office building sign in Van Nuys, "We're looking for a few good tenants who appreciate..."  Way to go!  Tenants are people too.

Lastly, Dr. Dani Santino, therapist in the above-mentioned tv show "Necessary Roughness", tells a patient's mother, "Our best hope is to convince Orson to check himself into a treatment facility that can help with his addiction."  Your turn.  This relative pronoun is correct because ....
... treatment facility is a thing.  You got it!

I hope that this was a posting that helped you understand the difference between these two relative pronouns and that you are people who will use them correctly.