What Are Articles?
Words used to define whether a noun is specific or not are referred to as articles. Here are some examples:
The last meal at the popular restaurant was extremely delicious.
With the article the in the above sentence, it is known that we had a meal at a popular restaurant and it was the meal we took there that was delicious, not a meal from anywhere else. 
However, in the following sentence: 
A meal at a popular restaurant is extremely delicious. 
Here, a general statement is made which tells that all meals are extremely delicious at any popular restaurant and at any time. 
In English, there are two types of articles: indefinite and definite. So, let's have a closer look at each of this in detail. 

The Definite Article
The word "the" comes to mind when a definite article is mentioned. This type of article confines a noun to a specific thing and leaves no room for ambiguity. For example, your colleague might ask "are you coming over for the reception?"  .Here, your friend is specific about a particular reception that you are well aware of. The definite article combines well with many nouns, whether they are singular, plural or uncountable. Here are other contextual examples of the use of the indefinite article "the." 
Could you please give me the pen?
Please show him the direction to the class; I'm very busy

The Indefinite Article
The other article form is the indefinite article, which expectedly doesn't give a direct clue as to the noun or subject in question. It takes two forms. The "a" form is used when a consonant begins the succeeding word, while the "an" form is used where a vowel begins the next word.
Thus, an indefinite article gives a general idea of a noun without any restriction. For example; A classmate might ask you "Will a class be taking place later today?" Here, you are being questioned on whether any class at all will be taking place later that day and not a specific class. 

Exceptions: Choosing A or An
Though you'll likely go with the "a" indefinite article before words that begin with a consonant, and "an" if a vowel begins the next word, there are certain exceptions to the rule. A common example is "hour." Although hour starts with a consonant - h, "an hour" and not "a hour" is used. This is because the "h" in hour is silently pronounced. Similarly, if a word begins with a vowel but is pronounced in the consonant form, the indefinite article "a" is used instead of "an." For example, we use a unicorn, not an unicorn. These rules also apply to acronyms and initialisms. For example "a UNHCR staff"  "an FAO unit."

Article Before an Adjective
Articles can be used before an adjective to modify nouns. However, the common sequence for this is article + adjective + noun. Consequently, a or an is used depending on the word that follows. Here are examples.
There is an old house next to John's.
Mercy delivered a brilliant speech.

Indefinite Articles with Uncountable Nouns
Some nouns are difficult to count and are thus referred to as uncountable nouns. Common examples are water, salt, sugar, information, etc. Since they are uncountable, indefinite articles "a" or "an" are not to be used with them. Indefinite articles can only be used for singular nouns. 
Alternatives like some and little are used instead of indefinite articles for uncountable nouns. For example, you say; 
"Please add some sugar to the tea" but not add a sugar to the tea. 
Some nouns can assume uncountable or countable forms, like glass, work, hope, etc., and should be treated accordingly when used with an article. 

Omission of Articles
In some cases, articles are intentionally omitted before some nouns. They are implied but not expressly present in sentences containing such nouns. Thus, there is also what is referred to as a "zero article." For example, articles are often omitted before nouns that refer to abstract ideas such as subjects and sports. E.g.
I'm very good at football, not I'm very good at the football. 
My friend was the best at chemistry back in college, not My friend was best at the chemistry back in college. 

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