Basic information
•To say ‘You know which I mean’, we put the before a noun. I’ve been to the dentist. (You know which one: one whom I visit regularly.) Have you locked the doors? (You know which ones I mean.) Could you pass the pen? (You can see the pen that I want.)
•When we can’t say ‘You know which I mean’, we put a/an before a singular countable noun . There’s a mosquito in the kitchen! I need a book.

•Put no article with a plural or uncountable noun that are not specific. He’s afraid of dogs.
She wants.

Common mistakes

•Don’t use a/an with plural or uncountable nouns. She collects stamps. (NOT ••• a stamps.) Your garden needs water. NOT…. a water.

•Don’t use the to talk about things in general. The does not mean ‘all’. (But, there are exceptions) Tortoises can swim very well. (NOT….. The tortoises . .. ) Diesel is expensive. (NOT…..The diesel … )
•Don’t use articles together with his, this, or other determiners. e.g. His work (NOT.. The his work) this pen (NOT… the this pen) a friend of yours (NOT a yours friend )
•Don’t use singular countable nouns alone, without an article or other determiner. We can say a rat, the the rat, my rat, this rat, any rat, either rat or every rat, but not just rat. (But, there are few exceptions) Give it to the boy. (NOT ….. boy) Neelam is a teacher. (NOT Neelam is teacher.)

Uses of a/an
A/an does not add much to the meaning of a noun – it is like a weak form of ‘one’. It has several common uses.

1 one person or thing
We can use a/an when we talk about one person or thing. There’s a police car outside. My sister’s married to a doctor. Annu lives in an old cottage.
2 anyone member of a class.
We can use a/an when we talk about anyone member of a class. A doctor must like people. (= any doctor) I would like to live in an old house. (= any old house)
3 classifying and defining
We can use a/an when we classify or define people and things – when we say what they are, what job they do, or what they are used for. She’s a doctor. I’m looking forward to being a grandmother. A glider is a plane with no engine. Don’t use your plate as an ashtray.
• When a/an cannot be left out
We do not normally leave out a/an in negative expressions, after prepositions or after fractions. Lend me your book. -I haven’t got a book. He mustn’t go out without an umbrella. three-quarters of a pound (NOT three-quarters of pound)
And we do not leave out a/an when we say how things are used . e.g. I used my shoe as a hammer. (NOT .•• as hammer.)
When a/an is not used: adjectives alone; possessives

A/an cannot normally be used with an adjective alone (without a noun). Compare: It’s a good book.
It’s good. (NOT It’s a good.)

The choice between a and an depends on pronunciation, not spelling.

We use an before a vowel sound, even if it is written as a consonant. an hour, an MP etc.And we use a before a consonant sound, even if it is written as a vowel. a uniform, a one-rupee coin etc.
Some people say an, not a, before words beginning with h if the first syllable is unstressed. an hotel (a hotel is more common) an historic occasion (a historic . .. is more common)

Sometimes we talk about things in general by using the with a singular countable noun. Schools should concentrate more on the child and less on exams.
This is common with the names of scientific instruments and inventions, and musical instruments. e.g.

Life would be quieter without the telephone. The violin is more difficult than the piano.
We can also generalise by talking about one example of a class, using a/ an (meaning ‘any’) with a singular countable noun. A baby goat can stand as soon as it’s born. A cow is a pet animal.
Note that we cannot use a/an in this way when we are generalising about all of the members of a group together. The peacock is in danger of becoming extinct. (N OT, A peacock) The sentence is about the whole peacock family, not about individuals.)
Do you like cats? (NOT, Do you like a cat?)

We usually leave out a/an after kind of, sort of, type of and similar expressions. What kind of girl is she? Have you got a better sort of phone?

Man and woman can be used in a
general sense without articles.e.g. Man is mortal. Not a man.

To talk about the seasons in general, we can say spring or the spring, summer or the summer, etc. There is little difference. London is lovely in (the) spring.
I like (the) winter best.
When we are talking about particular springs, summers etc, we are more likely to use the. I worked very hard in the summer that year.

We often use the + singular when we talk about musical instruments in
general, or about playing musical instruments. The violin is really difficult.
Who’s that on the piano?
But the is often dropped when talking about jazz or pop, and sometimes when talking about classical music. This recording was made with Rick Jhones on trumpet.
The names of illnesses and pains are usually uncountable, with no article, in standard British English. Have you had appendicitis?
A/an is used in a few cases such as a cold, a headache. I’ve got a horrible cold.
The can be used informally with a few common illnesses. I think I’ve got (the) flu.
I’ve got toothache again. Have you got a headache? She’s never had (the) measles.
American usage is different in some cases. I’ve got a toothache / an earache / a backache etc. Source: (Oxford University Press)