不定冠词
不定冠词有两个：a 和 an 。不定冠词就跟其他冠词一样，没有型态上的变化。至于要选用哪一个不定冠词，要取决于名词第一个字母或第一个音节的发音。当名词是子音开头，或是以 u 或 eu 开头（发音上接近「you」），就用 a；而当名词是以母音开头，或是以不发音的 h 开头，则用 an。
举例
- 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
初次提及某物时，会在名词前用不定冠词。有时特指群体中的一个成员时，也会用不定冠词。以下提供几个使用的时机和例句：
第一次提及某物时，用不定冠词。
举例
- Would you like a drink?
- I've finally got a good job.
- An elephant and a mouse fell in love.
特指群体的某个成员
在职业名前加不定冠词。
举例
- John is a doctor.
- Mary is training to be an engineer.
- He wants to be a dancer.
在单数的国籍或宗教名称前加不定冠词。
举例
- John is an Englishman.
- Kate is a Catholic.
如果没有特指任何一周的某个日子时，就在那个日子前用不定冠词。
举例
- I was born on a Thursday.
- Could I come over on a Saturday sometime?
用不定冠词指某件事的例子。
举例
- The mouse had a tiny nose .
- The elephant had a long trunk .
- It was a very strange car .
在 what 和 such 后的单数名词前用不定冠词。
举例
- What a shame !
- She's such a beautiful girl .
- What a lovely day !
在指单个人或物或者单个测量单位时，就用 a 来表示「一个」。在这些句子中，a 并不是不定冠词，但在文法上是正确的。这种用法强调了数量，也表达数字间的对比。
举例
- 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!