The indefinite articles a and an are used to introduce something that has not been mentioned before. The indefinite articles are not used before a plural noun. We use a before a word that begins with a consonant. We use an before a word that begins with a vowel, or a word that begins with a consonant but has vowel sound (e.g. hour, honour, etc)

 
We use a:

when we mention something for the first time

  • I saw a dog

before a word which begins with a consonant

  • There is a woman waiting for you.

before a word with a long sound of u

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

before the word one because one sounds as if it begins with a W (wun)

  • a one-way street, a one-eyed monster, a one-year course, a one-week holiday, etc
  • I have a one-way ticket to travel from one place to another, as I don’t intend to visit a place twice.
 

The indefinite article a also means one. We can use a or one as follow:

  • He keeps a/one dozen snakes as pets.
  • I have told you a/one hundred times to leave me alone!
 

Sometimes, it is better to use a instead of one.

  • She wiped up the vomit with a mop.
  • Better than: She wiped up the vomit with one mop.

  • He had a bath before he went to bed.
  • Better than: He had one bath before he went to bed.
 

We can use a before a proper noun.

  • Mr Brown called to ask when you are going to return the borrowed money.
  • I still remember it was on a December morning when I drove through the fog into a tree.
 
We use an

before a noun which begins with a vowel sound

  • They say an apple a day keeps the doctor away.

before a singular noun (person or thing) to mean only one in quantity

  • She’s an only child.

before a noun that is representative of a group, species, etc

  • An ostrich has only two toes on each foot.

before a noun that begins with a silent h

  • an hour, an honest man, an heir, an honour, an honourable man, etc
  • It is going to close in an hour’s time.
  • I can’t read your writing. Is this an h or what?

before abbreviations, some of which begin with a consonant

  • I have an X-ray on my lungs.
  • I saw an UFO hovering above my house.
  • He wanted to be an MP but was not elected to be one.