Unit 4 Astronomy: the science of the stars

4. he’s always the first to come and the last to leave.
5. now that you have grown up you can decide it yourself.
step 2 preparation
show some example sentences on the screen.
1 a tree has fallen across the road.
2 you are a student.
3 to find your way can be a problem.
4 smoking is bad for you.
5 “how do you do?” is a greeting.
6 what she said is not yet known.
7 that we shall be late is certain.
8 it’s certain that we shall be late.
what part does the underlined part serve as in each sentence? or find its subject in each sentence.
step 3 grammar
定義： 用作主語的從句叫主語從句，它是名詞性從句之一。引導主語從句的連接詞有：連詞that, whether；連接代詞who, what和which；連接副詞when, where, how和why。
1) 由what等代詞引起的主語從句：
       what the teacher said today was quite right.   老師今天說的話是十分正確的。
2) 由連詞that引起的主語從句：
        that they are badly in need of help is quite clear. 他們急需幫助，這是十分明顯的。
         但是這類句子在大多數情況下會放到整個句子的后部去，而用代詞it作形式主語。
   3) 由連接代詞或連接副詞以及whether引起的主語從句。
       whether they will sell the house is not yet decided.他們是否賣掉這所房子還沒有決定。
4) whatever, whoever也可引導主語從句。 
        whatever was said here must be kept secret.  這里說的話都應當保密。
        whoever makes mistakes must correct them. 凡犯了錯誤的人都必須改正。
   注意: 上述例句中的主語從句都是放在句首, 但有時為了使句子結構平衡, 避免 “頭重腳輕”, 常用 it 作形式主語, 而把從句放在后面。 例如:
　　it is a pity that she has made such a mistake.她犯了這樣一個錯誤, 真是遺憾。
這樣就構成了下面一些常用句型:
 1) it is + adj. / n. +從句
　 it is a pity/shame that... 遺憾的是……
　 it is possible that...  很可能……
　 it is unlikely that...  不可能……
 2) it +不及物動詞+從句
　 it seems/appears that...   似乎……
　 it happened that...           碰巧……
3) it + be +過去分詞+從句
　　it is said that...          據說……
　　it is known to all that...  眾所周知……
　　it is reported that...  據報道……
　　it is believed that...據信……;人們相信……
　　it is suggested that... 有人建議……
單個的主語從句作主語時謂語動詞用單數；由 and 連接的兩個主語從句作主語時，謂語動詞用復數。