The indefinite article (A, AN)

The indefinite article is generally used before a singular countable noun when it has not been mentioned before and does not represent a unique thing or person:

A man and a woman came in.

When the noun is mentioned again, the definite article the is used:

A man and a woman came in. The man was about 40 years old.

A is used before a word beginning with a consonant sound, but if the following word starts with a vowel sound, we use an:

Regina was wearing a dark skirt and an orange blouse.
My uncle is a university lecturer. (university starts with the vowel u but the consonant sound /j/)
He is an honest man. (honest starts with the consonant h but it is silent here)