不定式结构作主语
1、不定式前置
1) Derive [A] mineral resources from [B] sedimentary rock [C] is a [D] major modern industrial activity.
2) in the matriculation for postgraduate in a short time is realy a hard nut.
[A] To make high scores[B] To make low goal[C] To the high scores[D] Make the low goals
2、上述形式一般情况不多见，作为考试的规范英语，如果不定式较长，显得头重脚轻,则可由代词“it”作形式主语(形式主语“it”不能由“that”或“this”等其他代词代替)，而将不定式放到后面
It is really a hard nut to get high scores in the matriculation for postgraduate in a short time.
3) John admitted that it's always difficult.
[A] for him being on time[B] being on time for him
[C] for him to be on time[D] on time for him
4) It is difficult for me [A] to be concentrating [B] while someone [C] is using [D] a vacuum cleaner.
3、不定式后置的情况不仅限于动词是系动词，也适用于其他动词
It took us a great deal of efforts to finish that task.
It made him extremely happy to get contact with his family in Taiwan since they separated more than 40 years ago.
4、不定式结构所表示的动作是谁做的，即不定式的逻辑主语，通常可以在特定的上下文中能看出，也可以通过带“for +名词短语”辨出
It is quite important for us to read good books during the period of general review(总复习)。
It is not difficult for those who study maths well to pass the exam.
5、在某些形容词(如careless,clever,considerate,foolish,good,impolite,kind,naughty,nice,silly,stupid等)作表语时，不定式后可以加of来引导出其逻辑主语
It is very kind of you to help me./It is stupid of him to do such a silly thing.
间或也可用for + there to be 表示(而且there后面的不定式只能是to be)。
It's a great pity for there to be much trouble in the company.
