3．泛指某人或某物，但未具体说明何人、何物，如：

　 A boy is waiting for me.

Give me a pen, please.

2．说明某人或某东西属于哪一类，如：

　 This is a book.

　 He used to be an engineer.

1．用在单数可数名词前，表示一类人或事物，如：

　 A dictionary is a useful book.

　 An underground train can start and stop quickly.

3．如果不定冠词和名词前有其他词，不定冠词的形式仍取决于它后面第一个词的语音形式。例如an actress, a beautiful actress。

不定冠词的用法如下：

2．如果不定冠词后面第一个词以不发音的h开头，而h后面第一个音的音素是元音因素，就用an。例如hour, honest。

不定冠词用a还是an由后面一个词的首音(不是辅音字母)决定。以元音开头的词前用an，以辅音(包括半元音，不是元音字母)开头的词前用a。例如：

辅音字母开头：a tree, a university, a house, a European country, a one-eyed man

元音或半元音开头：an apple, an umbrella, an hour, an elephant, an opera

值得注意的是：

1．如果不定冠词后面第一个词以元音字母u开头就用a。例如university, useless, useful, unit, uniform。

5.　　　 Mum, I am so hungry. I want to eat ______. _______ will do.

4.　　　 She did better than I. There are ______ mistakes in her test than in my test

3.　　　 ______ child enjoys Christmas. They ______ have some presents.

2.　　　 There’s a red ball in the boy’s hand and an apple in his _______ hand.