By request (A request! How exciting!), here is a tutorial about “that,” “which” and “who.”
First, let’s tackle “that” vs. “who.” It is easy: Use “who” when you are referring to people, and “that” when you are referring to things. If you feel strongly about your pets, they can be “who” too.
Examples:
The man who gave me “The International” now lives in Richmond Hill.
Trish is the chicken who lives with us.
The cops shot the chimp that ripped off that lady’s face. (You could use “who” here, but I prefer not to give that level of respect to that particular chimp.)
The book that sits on my nightstand is some trashy romance novel.
Now let’s talk about “that” vs. “which,” which seems to be more difficult. THAT was the proper use of “which.” Use it only when you can use a comma before it. Otherwise, use “that.” In other words, “that” is a crucial part of the sentence and describes a particular kind of thing, and “which” begins a clause that could be thrown out if necessary.
Examples:
Heidi and John held a New Year’s Eve party that resulted in a five-alarm fire.
Heidi and John’s New Year’s Eve party, which resulted in a five-alarm fire, was the talk of the neighborhood.
See the difference? Many academics don’t. Now you are smarter than a 20th grader.
Observe and report 
Do they have New Year’s Eve fires in your country?
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Praise you for these grammar lessons Beth. I enjoy the updates on your animals and kids immensely as well.
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