The indefinite articles are a and an. We use them to show an unspecified noun, and not a special one, or when we are speaking in general.

A is used:

• before a word which begins with a consonant.

Example: There is a woman waiting for you.

• before a singular, countable noun.

Example: A banana was all I had for lunch.

• when we mention something for the first time.

Example: I saw a dog.

• before a word with a long sound of u

Example: a university, a uniform, a useful book, a European, a unique opportunity.
Example: It would be a unique opportunity to travel in space.

• before the word one because one is pronounced wun, as if it began with W.

Example: a one-way street, a one-eyed man, a one-year course, a one-day holiday, etc.
Example: I have a one-way ticket to travel from one place to another as I don't intend to visit a place twice.

An is used:

• before a noun which begins with a vowel.

Example: They say an apple a day keeps the doctor away.

• before a word which begins with a vowel sound or a silent h.

Example: an hour, an honest man, an heir, an honour, an honourable man, etc.
Example: An hour passed and she still hadn't arrived.