不定冠詞
不定冠詞有兩個：a 和 an 。不定冠詞就跟其他冠詞一樣，沒有型態上的變化。至於要選用哪一個不定冠詞，要取決於名詞第一個字母或第一個音節的發音。當名詞是子音開頭，或是以 u 或 eu 開頭（發音上接近「you」），就用 a；而當名詞是以母音開頭，或是以不發音的 h 開頭，則用 an。
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!