e.g: To finish the building in a month is difficult.
To do such things is foolish.
To see is to believe. (对等)
注: 1). 不定式作主语时,谓语用单数
(1)It is/was +adj.+of sb. to do…
(2) It is +adj.+for sb.+to do…
It is easy / difficult / hard / foolish / unwise / right / wrong / unnecessary
(3) it is +a +名词+ to do...
It is a pity / a pleasure / a pleasant thing / one’s duty / an honor / a shame / a crime / no easy job… to do
It takes (sb.) some time / courage / patience …to do…
It requires courage / patience / hard work… to do…
*注意: probable 和 possible 均可作表语，但possible可以用不定式作真实主语， 而probable不能用不定式作真实主语。
It is probable for him to come to the meeting.(错)
It is possible for him to come to the meeting.<济南专治癫痫医院/p>
It is possible / probable that he will come to the meeting.
主语是以aim duty hope idea intention plan job suggestion wish purpose task 等为中心词的名词词组 或以 what 引导的名词性从句表示，后面的不定式说明其内容, 不定式作表语常表示将来或现在的动作或状态。
eg :My idea is to climb the mountain from the north.
Your mistake was not to write that letter.
What I would suggest is to start work at once.
Worried about the journey, I was unsettled for the first few days.
Well-known for their expertise, his parents’ company …..
Confused by the new surroundings, I was hit by the lack of fresh air.
Exhausted, I slid into the bed and fell fast asleep.
过去分词作状语：过去分词作状语时，说明动作发生的背景或情况，其等同于一个状语从句。vt 过去分词作状语时与主句主语构成被动关系，表示被动和完成，vi 过去分词表示状态或动作的完成。
Heated , water changes into steam .
The professor came in, followed by a group of young people .
1 作原因状语，等于as / since / because 引导从句
Moved by what she said ,we couldn’t help crying . = ( As we are moved by what she sa癫痫病好的治疗方法id …
2 作时间状语，等于when 引导时间从句，如果分词表示的动作与谓语的动作同时发生，可在分词前加when/ while / until 等使时间意义更明确。
When heated , water can be changed into steam .
Seen from the hill ,the park looks very beautiful .= ( When the park is seen from the hill…
3 作条件状语等于 if / whether 引导从句
Given more attention , the cabbages could have grown better .= ( If they have been given more attention ….
Compared with you , we still have a long way to go = ( If we are compared with you …
The actress came in , followed by her fans .
She sat by the window , lost in thought .
Much tired ,he still kept on working .=(Although he was tired ,) he ….
6 独立主格结构： 当分词的逻辑主语不是主句主语时，分词可以有自己独立的逻辑主语，这种结构称为独立主格结构。常用来表示伴随情况。
The boy rushed into the classroom , his face covered with sweat .
All things considered ,your article is of great value than hers .
Rewrite with proper conjunctions
Example : United we stand, divided we fall.
If we are united, we wi丽水癫痫病治疗疗法ll stand, but if we are divided,we will fall.
1 Asked what had happened, he told us about it.
→When he was asked what had happened, …
2 Well known for his expert advice, he received many invitations to give lectures.
→Because he was well known for his expert advice, …
3 Given more time, we would be able to do the work much better.
Eg:They kept the door locked for a long time.
Keep your mouth shut and your eyes open.(谚语：少说多看)
Don't leave such an important thing undone.
Don't leave the windows broken like this all the time.
eg: I have had my bike repaired.
The villagers had many trees planted just then.
Eg:I had my wallet stolen on a bus last month.
The old man had his leg broken in the accident.
He h北京哪医院治癫痫好ad his leg broken in the match yesterday.(MET1986)
They managed to make themselves understood in very simple English.
I raised my voice to make myself heard.
三、过去分词用在感观动词watch,notice,see,hear,listen to ,feel,find等后面。如：
When we got to school,we saw the door locked.
We can hear the windows beaten by the heavy rain drops.
He felt himself cheated.
The managers discussed the plan that they would like to see carried out the next year.(NMET2000)
The boss wouldn't like the problem discussed at the moment.
I would like my house painted white.
I want the suit made to his own measure.
I wish the problem settled.
The thief was brought in with his hands tied behind his back.
With many brightly-coloured flowers planted around the building ,his house looks like a beautiful garden.