information about the Articles

An article is a word that combines with a noun to indicate the type of reference being made by the noun. The three main articles in the English language are the, an and a. An article is sometimes called a noun marker, although this is generally considered to be an archaic term.  It is sometimes wondered which part of speech articles belong to. Since articles modify nouns, either alone or in combination with an adjective, they are sometimes classed as  adjectives. However, some linguists place them in a different category, that of determiners.  Generally English has four types of articles: The use of these articles depends mainly on whether you are referring to any member of a group, or to a specific member of a group:  
Definition of Articles:

The articles in English are the (definite article), a, and an (indefinite articles). Articles define a noun as specific or unspecific. There are only three articles: the, a and an. They are very small words which cause very large problems if used incorrectly.

Uses of Articles
Articles are used to differentiate between things or ideas – usually expressed by nouns. The speaker/writer may be referring to a specific thing or idea, or a general one.
We use the to refer to specific nouns, either singular or plural.
Please hand me the book that’s on the table.
Please hand me all the books that are on the tables.
A and an are used to refer non-specific nouns.
Please hand me a book; any book will do.
Please hand me an autobiography; any autobiography will do.
Articles should be placed just in front of the noun they’re modifying. However, if the noun is also being modified by one or more adjectives, the article should go in front of the adjective(s).
Please fetch me a sweater.
Please fetch me the brown sweater.
Please fetch me the fuzzy blue sweater.
An should be used before any noun – or any adjective modifying the noun – which begins with a vowel or a voiceless H.
He is a man.
He is an ugly man.
He is an honest man.
Articles can also be used when emphasizing a point.
He had a hard time with the exam.
He had the hardest time with the exam.
Obviously, using the definite article the makes the point very clear.

Definite Article: The

The definite article refers to a noun, either in the singular or plural. It can be used with uncountable nouns. The refers to a specific thing or quantity. It should be placed before the noun, or before the adjective which directly modifies the noun.

"The teacher I spoke to at the school was very friendly." - here we are talking about one specific teacher (the one I spoke to).
Indefinite Articles: A / An
The indefinite articles are used when we are referring to an unspecified thing or quantity. We use them when we don’t know (or don’t care) which thing we’re talking about.

"I want buy a new car." - there are many types of car. It does not matter yet which type I want.
Compare this sentence with:
"I want to buy the car we looked at yesterday." - we use the here because we are talking about a specific car (the one we looked at).

Example of some common errors in the use of articles

1-  (W) Aeroplane is the fastest means of travel that is in common use.
(R) The Aeroplane is the fastest means of travel that is in common use.
Note:- The is used before a singular noun to express what we call ‘the generic
singular’, i.e. the one thing mentioned is taken to represent all of that kind.
The lion is a noble animal. (correct)
An exception to the above rule is the noun man, when it is used to demote the human
race as a whole.
Man does not live by bread alone. (correct)
2. (W) I have worked hard all the day

(R) I have worked hard all day.
Note:- The is left cut of the expressions all day and all night but it should be used in
similar adverbial expressios for other divisions of time: all the morning, all the
afternoon, all the evening, all the week.
It has been reaining all the morning. (correct)
Americans often say all morning, all week etc. This is now frequently heard in
England also, and is becoming normal British usage. (F. T. Wood).