fian's update

Articles — both definite and indefinite

Before a singular noun which is countable when it is mentioned for the first time and represents no particular person or thing.
"A horse is a noble animal."
Before a singular countable noun which is used as an example of a class of things.
"A book is something you read."
With a noun complement, including names of professions.
"She is a doctor, he became a famous actor."
In certain numerical expressions.
"a dozen, a hundred"
In expressions of price, speed, ratio.
"60 miles an hour, 4 hours a day, 30p a box"
With "few" and "little"
"a few people, a little sugar"
In exclamations before singular, countable nouns. "What a pity! What a sunny day!"
It can be placed before Mr., Mrs, Miss, Ms + surname.
"a Mr. Brown phoned today."

Not Used

Before plural nouns.
"Horses are noble animals."
Before uncountable nouns
"Milk is good for you."
Before abstract nouns.
"Fear is natural."
Before names of meals except when preceded by an adjective.
"We had a late breakfast and decided to miss lunch."

Before nouns of which there is only one.
"The earth is round."
Before a noun which has become definite as a result of being mentioned a second time.
"We saw a good film last night. It was the film you recommended."
Before a noun made definite by the addition of a phrase or clause.
"The woman dressed in black."
Before a noun which, by reason of locality, can represent only one particular thing.
"There’s a bee in the kitchen."
Before superlatives and "first", "second" and "only"
"The longest river in the world."
Before singular nouns used to represent a class of objects.
"The donkey is a very obstinate animal."
Before an adjective used to represent a class of persons.
"That tax hurts the rich."
Before names of seas, rivers, chains of mountains, groups of islands and plural names of countries.
"the Pacific Ocean, the Thames, the Andes, the West Indies, the Netherlands"
Before musical instruments.
"She plays the piano"

Before countries, towns, proper names.
"Charlie lived in Chicago in America"
Before abstract nouns.
"Jealousy can be dangerous."
Before parts of the body and articles of clothing — these normally prefer a possessive adjective.
"Lift your left arm."
"He removed his hat."