Similar to count and noncount nouns, definite and indefinite articles can be a trouble spot for ESL learners. Rules vary from one language to another in regards to the usage of the definite versus indefinite, so some earnest practice with its rules in English would be of great advantage for the TOEFL exam.
In general, when speakers and writers do not have a specific person, place, or thing in mind, the corresponding nouns are known as non-specific and are often preceded with the indefinite article: "a" or "an" in the singular. Often times, too, a noun is definite when a speaker mentions it the first time and then definite ("the") from there on out.
What a fascinating story you just told!
Yes, the story was very exciting.
In regards to definite articles, we often use "the" when the speaker, listener, or writer knows the specific person, place or thing that's being discussed. "The" is used both for noncount nouns and singular and plural nouns that fall under the "definite" category.
The chowder we had at noon was fantastic.
The artwork is 100% authentic to the African region.
Definite articles are also used to describe something special, or unique.
Examples of unique nouns would be: the moon, the sun, the Empire State Building, the Big Dipper, etc...
Definite articles have further usages as seen in the following ways:
Public places: the library, the movies
Specific names of geographical places: the Great Lakes, the Nile River, the Amazon
Countries: the U.S., the United Kingdom
Overall, the definite and indefinite articles take some time to get used to in the English language. Our advice would be to memorize well-known definite articles first (countries, places, etc...) and to pay close attention to how indefinite articles and definite articles are used in conversation in both TV and film.
Remember: If you are not a native English speaker, keep in mind that these rules are probably different than in your native language. Keep these rules close to you so that come test day you are using a, an, some and the correctly on both the speaking and writing section.