A and An - Indefinite Articles

We use indefinite articles a and an when referring to living creatures and things that we talk about for the first time or do not know too closely.

A — we use it with nouns beginning with a consonant:

• a car
• a table

An — we use it with nouns beginning with a vowel:

• an apple
• an elephant
• an injury

Articles a, an mustn't be used with plural nouns:

• There are an elephants in the zoo.

Plural nouns appear alone or are preceded by words such as some or any.

some— used in positive sentences
any — used in questions and negative sentences

Examples:

Singular

• I've got a pen.
• Have I got a pen.
• I haven't got a pen.

Plural

• I've got (some) pens.
• Have I got (some) pens.
• I haven't got (any) pens.