What" is usually a question word.
Examples:
"What are you doing?"
"What are these?"
"What is your idea?"
"What" can also be used with the meaning of "the thing or things".
Here are some examples:
"What you need is a good night sleep." (= The thing you need is a good night sleep.)
"We don't know what to do." (= We don't know which things to do.)
"You know what to do." (= You know which things to do.)
"That" is used to describe and define the thing you are talking about.
For example:
"I want the pen."
"Which pen?"
"I want the pen that is on the desk."
Here's another example:
"I saw that girl."
"Which girl?"
"The girl that you invited to the party."
Now, regarding your specific sentence:
"The book what I need" would mean "the book which thing I need."
This doesn't make sense, does it?
On the other hand: "The book that I need" makes perfect sense, because you are giving a description of the book. - See more at: http://www.helping-you-learn-english.com/difference-between-what-and-that.html#sthash.GSOg7JPb.dpuf
July 30, 2015
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What is a question word.
You would say "What is the difference..."
That is a statement.
For example you would ask,
What is a dog?
I would respond by pointing to a dog and saying
That is a dog.
July 30, 2015
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That = ese/esa/eso/aquel/aquella/aquello
What = Qué/lo que
¿Qué quieres? No sé lo que quiero. What do you want? I don't know what I want.
Me gusta esa/aquella casa. I like that house.
July 30, 2015
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"that" and "what" - they have 100% difference
Some of context:
"that" is mean you knew something
"what" is mean you don't know something but you want to know
what is this?
That is the book
July 30, 2015
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