1. Articles: There are three articles in English—a, an and the. A and an are called indefinite article.The is the definite article. An article is placed before a noun. If there is an adjective before a noun, the article is placed before the adjective:

a train,
a fast train,
an incident,
an unusual incident
Note: We can never use a singular count noun alone, that is, without a/an/the/my/some/any etc.

2. A/an: Singular count nouns take the indefinite article a/an with them:

a ball
an egg
a dog
an elephant

Uncount nouns do not generally take an article with them. we do not generally say

a milk
a beauty
a wisdom
for milk, beauty, wisdom cannot be counted.

3. We use a with singular count nouns beginning with a consonant sound:

a girl
a map
a university
a union
a one-sided affair
a one-rupee note

Note: That the words university, union, and one begin with a vowel but no a vowel sound. University and union begin with the yoo sound while one begins with the w sound.
Well-known words which begin with a vowel but take a with them are:

European
uniform
union
unit
universal
usual
useful
eau-de-cologne

4. An: An is used before words beginning with a vowel sound:

an umbrella
an opportunity
an honest boy
an honourable person

The letter h in honest and honourable is not sounded. Common words in English which begin with an unsounded h are:

heir
heiress
honest
honorary
honourable
hour
hourly

5. In abbreviations, if consonants begin with a vowel sound, they take an before them:

an M.P.
an S.P.

But if consonants begin with a consonant sound, they take a before them:

a Ph.D.
a B.Ed.