The definite article the is used in several different ways
One use of the is with nouns (singular or plural, countable
or uncountable) that are specifically identified because the
listener or reader knows (or can assume) that only one
specific noun is being talked or written about.
Another use of the happens when a non-specific noun
is mentioned more than one time.
A third use is related to the second: when a plural noun
is non-specific, either some, a number or other quantifier,
or no article is used when the noun is mentioned the first
time, but the is used after the first time.
Celina is wearing earrings. The earrings are small and
round and made of gold.
Someone sent Anna flowers yesterday. The flowers
were delivered just before noon.
George and Helen have three children. The children are
studying at universities in Missouri.
I bought a pair of shoes yesterday. I couldn't resist
buying the shoes because they were on sale for half price.
Betsy showed us some some photographs of Switzerland.
The photographs were taken on Betsy's vacation last year.
Julia bought a skirt and some shoes. The skirt and the
shoes are both navy blue.
She knew that we had questions about her report.
She asked us to send her the questions by e-mail.
Uncountable nouns can also be used in a way that is similar
to what was noted above:
There's water all over the kitchen floor! I think the water
is coming from the bottom of the refrigerator!
I see a lot of smoke. Do you think the smoke is from
a forest fire?
Fred bought several packages of graph paper. He'll use
the graph paper in his geometry class.
I bought two quarts of ice cream. We'll have the ice cream
for dessert this evening.
If your sauce is too thin, add one or two teaspoons of
cornstarch. The cornstarch will make the sauce thicker.