舉例
a boy
an apple
a car
a helicopter
an elephant
a big elephant
an itchy sweater
an ugly duck
a european
a university
a unit
an hour
an honor
初次提及某物時，會在名詞前用不定冠詞。有時特指群體中的一個成員時，也會用不定冠詞。以下提供幾個使用的時機和例句：
第一次提及某物時，用不定冠詞。
舉例
Would you like a drink?
I've finally got a good job.
An elephant and a mouse fell in love.
特指群體的某個成員
在職業名前加不定冠詞。
舉例
John is a doctor.
Mary is training to be an engineer.
He wants to be a dancer.
在單數的國籍或宗教名稱前加不定冠詞。
舉例
John is an Englishman.
Kate is a Catholic.
如果沒有特指任何一週的某個日子時，就在那個日子前用不定冠詞。
舉例
I was born on a Thursday.
Could I come over on a Saturday sometime?
用不定冠詞指某件事的例子。
舉例
The mouse had a tiny nose .
The elephant had a long trunk .
It was a very strange car .
在 what 和 such 後的單數名詞前用不定冠詞。
舉例
What a shame !
She's such a beautiful girl .
What a lovely day !
在指單個人或物或者單個測量單位時，就用 a 來表示「一個」。在這些句子中，a 並不是不定冠詞，但在文法上是正確的。這種用法強調了數量，也表達數字間的對比。
舉例
I'd like an orange and two lemons please.
I'd like one orange and two lemons please.
The burglar took a diamond necklace and some valuable paintings.
I can think of a hundred reasons not to come.
I need a kilogram of sugar.
I need one kilogram of sugar.
You can't run a mile in 5 minutes!