a/an （用于未曾提到的可数名词或者单数名词，表明一类人或事物以区别于它类，只说明是这一类的东西，但未说明是这一类的什么。This is a （good）book. 这是（一本）（好）书。具体什么书，不知道。就算加了good也没事，还是泛指，因为对方不知道这个到底是那本书 ）
有以下句子：
war is a drug（一部电影开头，简述下战争）......暂时就一个句子吧，其他应该也差不多。求老师指教。
一、泛指与特指
1、不定冠词和复数名词都可以表示泛指，即指一类或其中的任意一个/些或者某个/些不知名的人或物。如：
I wish I had a dog of my own. ( a dog指的是种群中的任意一个。)
Pandas are rare animals. (pandas指的是任何一群)
2、 定冠词表示特指： 包含以下两个方面：
明确性
The panda is a rare animal. (the panda表示的是一只明确的动物。)
比较：A dog charged at me when I was on my way home last night. (a dog尽管有所指，但只是种群中一只不明确的动物。)
唯一性
1. 自然界唯一
The moon appears at a particular time in its cycle of phases: (the moon自然界唯一的卫星。)
2. 心目中唯一
The desk is the only one where he lives. (the desk是说话人心目中唯一的物件。)
The panda gave birth to a healthy baby last night. (the panda是说话人心目中唯一 的动物。)
3. 语言环境中唯一
Would you pass me the cup? (the cup在语言环境中有别于其他任何cup。)
That’s the dictionary I’m looking for. (the dictionary是语言环境中唯一的。)
而、可数名词前不定冠词的用法：
1. 表示一类
A canal is a “channel cut through land for use boats or ships or to carry water to fields for irrigation”.
This is a dictionary, not a an encyclopedia(百科全书).
2. 表示一个（one的弱化）
I’d like a cup of coffee. 比较：I only want one cup of coffee and you’ve given me two.
I have got a brother and a sister. 比较：He has one brother, but two sisters.
3. 表示主语的工作或职业身份
John is a carpenter and his wife is a waitress.
注：但是当表示职位并且是唯一的职位时就不能加a/an。如：
He was elected chairman of the committee.
As president of the United States, Bill Clinton was younger than any of his predecessors.