Is it your brother？ 是你弟弟吗？ I didn't know it was you. 我不知道是你。
“Oh？ who was it？” Mr. Black asked. “哦？是谁？”布莱克先生问。
（3）it作无人称代词时，通常在句中作形式主语，用来表示自然现象、时间、距离等。 ① It is nine sharp now. 现在是九点正。 ② It is very hot today. 今天天气很热。 ③ It is far to the airport. 到飞机场很远。
④ It is about 17ff million kilometres from the earth to the sun. 从地球到太阳约一亿七千万公里。 2.引导词it
（1）作形式主语，代替由不定式（或不定式复合结构）、动名词或从句表示的真正的主语。
As a student， it is bad manners to come late to class. 作为一个学生，上课迟到是没有礼貌的。
It is possible that he will find the lost stamp. 他有可能找到遗失了的邮票。
（2）作形式宾语，代替其后所说的真正的宾语。真正的宾语是由不定式、动名词或从句表示的。
This has made it necessary for agriculture and industry to develop very quickly. 这就使得工农业必须飞速发展。
You may think it strange that anyone would live there. 你也许感到奇怪，居然有人愿意住在那儿。
The invention of radio has made it possible for mankind to communicate with each other ever a long distance. 无线电的发明使人类有可能进行远距离通讯联络。 3.用于强调结构中的it
为了强调句子的某一部分（通常是主语、宾语或状语），常用it作先行代词，这种 结构的句型是：It+is（was）+被强调的部分+that（或who）+其他部分，如果强调的 部分是人时须用who，而不用that.
It is the force of gravity that makes heavy things fall toward the ground. （强调主语） 是地心引力使重物落向地面。
It is from the sun that we get light and heat. （强调状语） 正是从太阳那里我们得到了光和热。
It was the goals eyes that he had seen in the darkness. 他在黑暗中看见的是山羊的眼睛。（强调宾语）
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