it的基本用法
(1)it 代替前文已提到过的一件事物。
The vase is valuable.It's more than 200 years old.那个花瓶很珍贵，它有200多年的历史。
(2)当说话者不清楚或没必要知道说话对象的性别时，也可用 it 来表示，起指示代词的作用。
—Who is making such a noise?
是谁发出这样的吵闹声？
—It must be the children.
一定是孩子们。
(3)it 无指代性，常用作没有具体意义的主语，出现于表示天气、气候、季节、温度、时间、地点、距离、环境情况等意义的句子中。
It is six miles to the nearest hospital from here.这里离最近的医院有六英里。
It was very cold.It snowed and grew dark.
天很冷；下着雪，天变黑了。
It was very noisy (quiet) at the very moment. 此刻非常吵闹/安静。
(4)it 也常用来表示一般的，笼统的情况。
It's awful—I've got so much work but I don't know where to start.
糟透了——我有这么多工作要做，却不知从何开始。
(5)it 还可在介词后占有宾语的位置，然后通过 that 引出一个介词宾语从句。
You may depend on it that they will support you.你放心好了，他们会支持你的。
(6)it可用在主语的位置，代替不定式、动名词或从句作形式主语，使句子保持平衡，避免头重脚轻。
It's important for us to master the computer skills.
对我们来说，掌握计算机技能十分重要。
It's no use trying to lose weight without keeping your diet.
想减肥却不节制饮食是没用的。
It is said that the meeting will not be held.
据说会议不开了。
(7)it也可用在宾语的位置，代替不定式、动名词或从句作形式宾语，使句子结构更加紧凑。
I found it quite pleasant to work with him.
我发现和他在一起工作很愉快。
I don't think it worthwhile taking such trouble. 我认为不值得费这么大的劲。
We took it for granted that they would like the idea.
我们想当然地认为他们会喜欢这个主意。
(8)用在某些词组中，没有特殊意义。
That's it. 那正是我想知道的。
At last we've made it. 我们终于成功了。