Basically, an article is an adjective. Articles are considered a special category of adjectives. Like adjectives, articles modify nouns. English has two articles: the and a/an. The rule of thumb is an article must be followed by a noun because in English it is not possible to refer to a noun without an adjective modifying it.
Incorrect: There are so many things on table. Table is large. I see book on table.
Correct: There are so many things on the table. The table is large. I see a book on the table.
As articles a, an, the are one type of adjectives, they can be replaced with other kind of adjectives, such as demonstrative adjectives: this, that, these, those; possessive adjectives: my, your, her, his, its, our, their.
There are so many things on my table. Your table is large. I see his book on your table.
The indefinite article a and an modify single nouns. The meaning of the article a is similar to the number one, but one is stronger and gives more emphasis.
I heard it an hour ago.
I heard it one hour ago and I hear it now again.
An is used before all nouns that begin with a vowel sound. (The vowels are a, e, i, o, u)
an apple, an opinion, an unusual day
Also, use an before a noun that begins with an h if the h is not pronounced.
an hour, an honor (the h in hour and honor are not pronounced)
but a hospital (the h in hospital is pronounced)
a unicorn, a usual day, a union
Articles also modify adjectives before nouns:
I have had a delicious meal.
This is an authentic remark.
The angry man raised his voice at the clerk.
To learn more about English articles, see English Articles, and Do and don't use articles Guide.
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