"That is a book you borrowed
last month."
"That is the book you borrowed
last month."
Note that both are correct! In the first example, the
second speaker implies that the other person may have borrowed
more than one book last month. In the second example, there is
no doubt: last month the first person borrowed one book
from the second person.
Plural words:
"Maybe he steals cars for a living."
"Maybe he wears disguises; thieves can do that."
Words signifying things that cannot
be counted easily. Words such as "sand, milk,
fruit" and other things that are measured in quantity can
be preceded by a measurement (eight cans, a gallon, a pound):
"We found sand in the car." ("the sand"
if it was some specific sand that was being discussed).
"Where was the sand? In the trunk or on the carpet? Did
you find anything else?"
"We found beer in the trunk."
"How much beer?"
"Eight cans of beer."
Words signifying abstract concepts,
ideas, or subjects:
"I'm studying biology this year."
"What are you doing in philosophy?"
"We're studying the history of knowledge." (a specific
type of history, but not "the knowledge"--knowledge
is a general concept).