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)
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.
- A 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.
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.