Article

In English grammar, an article is a kind of adjective or determiner used as a demonstrative adjective before noun. An article is a word which is usually used together with a noun to indicate the definiteness or complete meaning of the noun refered to. Only the three words, "a", "an", and "the" are articles.

In general, the use of a noun in general sense means its use to cover a whole class.

The tiger is fierce

The lion is majestic.

Beauty is truth.

Love is blind.

While the use of a noun in particilar sense means its use to cover a single object or an example of a class.

The book on the table is a dictionary.

The book on the table is a novel.

The car which I have bought is a saloon.

The car which I have bought is a compact MPV.

The boat which I have bought is a kayak.

The boat which I have bought is a canoe.

There are two types of articles to complete the meaning of the noun refered to. They are the indefinite article and definite article. The indefinite article is "a" or "an", and the definite article is "the". The main different between indefinite article and definite article is the type of noun refer to. Usually, an indefinite article is used when the specified noun does not refer to something specific. While a definite article is alway used when the specified noun refers to something specific. 

Usually, either the indefinite article "a" or "an", or the definite article "the" is usually used before a noun. For example,

1. When a common noun is used in the singular sense.

   A camera is useful on a holiday.

   A camera is useful on a tablet.

   A camera is useful on a phone.

   The bee is an example of industry.

   The ant is an example of diligence.

2. When an abstract noun is used in the particular sense.

   Patience is a virtue.

   Prudence is a virtue.

   Justice is a virtue.

   Fortitude is a virtue.

   Temperance is a virtue.

   This is a difficulty.

   This is a challenge.

   The honesty of my employee is doubtful.

   The honesty of the transaction may be doubtful.

3. When a collective noun is used in a general or particular sense.

   The pride of lions are cats.

   I am excited every time I get close to a pride of lion.

   The group of words is called a phrase.

   He is attacked by a pack of wolves

4. When a proper name is used in a specific sense.

   I received a call from a Mr White.

   The Himalayas is a mountain range in Asia.

However, both the indefinite article "a" or "an" and the definite article "the" are sometimes not used before a noun in some cases.

Besides, both the indefinite article "a" or "an" and the definite article "the" can also be used before other parts of speech like adjective in some cases.

We should help the poor.