1 one 一（个）: A cup of coffee, please. 请给我一杯咖啡。We've got an apple, a banana and two oranges. 我们有一个苹果、一根香蕉和两个橘子。

2 used when you talk about one example of sth for the first time （用于首次提及的事物）: I saw a dog chasing a cat this morning. The cat climbed up a tree. 今天早上我看见狗在追猫。那只猫爬了上树。Have you got a dictionary (= any dictionary)? 你有词典吗？

3 used for saying what kind of person or thing sb/sth is （用以表达何种人或物）: He's a doctor. 他是医生。She's a Muslim. 她是穆斯林。You are a clever boy. 你是个聪明的孩子。‘Is that an eagle?’‘No, it's a falcon.’ “那是鹰吗？”“不，是隼。”

4 (used with prices, rates, measurements) each （与价格、比率、度量衡单位连用）每，每个: I usually drink two litres of water a day. 我一天通常喝两升水。twice a week 一星期两次He was travelling at about 80 miles an hour. 他当时正以每小时约80英里的速度行进。

5 used with some expressions of quantity （与某些表示数量的词连用）: a lot of money 很多钱a few cars 几辆汽车

6 used when you are talking about a typical example of sth （用以指某物的典型例子）: An elephant can live for up to eighty years. 大象可以活到八十岁。 You can also use the plural in this sense Elephants can live for up to eighty years. 这个意义也可用复数：Elephants can live for up to eighty years.