@ElliotGreen,
Punky: "The car has broken down."
The use of 'the' in "the car" indicates a specific car; but which specific car?
Ohhhhh....The car THAT I had just bought has broken down.
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Elliot: "We use 'which', generally when we are giving extra information about a clause, and where the clause itself is independent of the extra information"
...and use 'that' when the clause defines more specifically "the pile of wood" you are referring to.
'I added a log to the pile of wood THAT I use as building material'
"'I added a log to the pile of wood THAT I use to build things.'"
What YOU may know ((I have lots of wood.... Maybe I use other piles of wood as fire wood.' is NOT known to the reader. The reader wants to know ,'what pile of wood' are you talking about?