Language

b. when the subject is a to-infinitive.
a. (preferable) it is hard to master a foreign language. =
    (correct) to master a foreign language is hard.
b. (preferable) it is important to know how to use a computer. =
    (correct) to know how to use a computer is important.
c. when the subject is a v-ing form.
a. ( preferable) smoking is difficult to stop. =
    (correct) it is difficult to stop smoking.
b. (preferable) travelling to beijing takes up a lot of my time. =
    (correct) it takes a lot of my time travelling to beijing.
tip:
we usually use the empty subject it with a clause or to-infinitive (preferable), but with the v-ing form we prefer the real subject at the beginning.
ш. it can also be used before seem, appear, happen, chance, turn out and prove as the empty subject of a sentence.
a. it seems that he speaks two languages. =
   he seems to speak two languages.
b. it happens that my new neighbor comes from my home town. =
  my new neighbor happens to come from my home town.
iv. we can emphasize the subject, object, or adverbial in a sentence by using it +be+ the word/phrase you emphasize+ a clause introduced by that or who.
the part we emphasize:
1. it is prof. li who teaches us english. （emphasize the subject）
2. it is english that prof. li teaches us. （emphasize the object）
3. it was in shanghai that i saw the film. （emphasize the adverbial）
multiple choices：
1、___d___ is a fact that english is being accepted as an international language. (nmet1995)
 a. there    b. this        c. that     d. it
2.i like___c___ in the autumn when the weather is clear and bright.  （04全國）
    a this b that  c it    d one
3、the chairman thought ___b___ necessary to invite professor smith to speak at the meeting. （05全國卷1）
    a.that   b.it     c.this  d.him
brainstorming:
it is ________________that keeps us healthy.
【設計說明】先簡單復習“it” 的功能。在英語中為了避免頭重腳輕，我們經常把主要信息放在句子后面部分，而用it作形式主語。然后講解it的用途 ：當 真正主語為從句或不定式時，最好用it作形式主語，當真正主語為v-ing時，還是把v-ing主語放句首。之后配上一定練習和造句進行鞏固。

step 4  other important cases of noun clauses:
i.同位語從句和定語從句的區別:
1.定語從句是先行詞的修飾語，它不涉及先行詞的具體內容。定語從句中that不但起連接作用，而且在定語從句中充當一個句子成分，充當從句的賓語成分時可省略。