|We use the indefinite articles a / an|
|1. to refer to something or someone for the first time in a conversation:
I've got two children – a boy and a girl. The boy is 7 and the girl is 4.
2. before professions and job titles:
a nurse a plumber a politician a marketing executive
3. with some expressions of quantity, amounts and frequency:
a pair of jeans a few minutes twice a year 50 miles an hour
4. with expressions with "What ... !":
What a great dress! What a mess! What a shame!