"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).