网友提供的这3个句子，各具特色，结构不尽相同。我分别解答如下：

It is to be regretted that he should have committed this foolish act.       

他做出这样的愚行来真是遗憾。

【答】

1. 本句It是形式主语；that he should have committed this foolish act是真正主语。

2.  is to be regretted：不是系表结构，而是表将来的be to do结构，表示“将来注定做某事；不可避免做某事”。

3. should have committed：在这里不表示“过去本该做某事而实际未做”，而是“在过去竟然做了某事”，should在此表达一种“惊讶”的语气。所以翻译为“他竟然做了这么愚蠢的事情”。

It is a matter to be regretted that he should have failed again in the examination.       

他竟然又一次考试不及格，这是一件令人遗憾的事。                           

【答】

1. 本句It是形式主语；that he should have failed again in the examination是真正主语。

2.  It is a matter 是“主系表”结构；to be regretted作定语修饰matter：一件令人遗憾的事。

3. should have failed：在这里不表示“过去本该做某事而实际未做”，而是“在过去竟然做了某事”，should在此表达一种“惊讶”的语气。所以翻译为“他竟然又一次考试不及格”。

It is much to be regretted that you can't join us on the trip.

你不能和我们一道去，肯定是非常遗憾的。

【答】

这个句子比较特殊一些。其实，它是第一句It is to be regretted的变式，只是在不定式前，加了程度状语much而已。又如：

The mistake is much to be regretted. 这个错误将会很令人遗憾。

I told her she was right, and I knew it was much to be regretted, but still it was not to be helped.

我对她说她是正确的，我知道我的这种想法是非常令人遗憾的，可是这又有什么办法呢？

It is much to be regretted that this advise, subsequently repeated, was not taken. 非常遗憾的是, 后来再度提出的这项建议没有被接受。

1. 本句It是形式主语；that you can't join us on the trip是真正主语。

2.  is nuch to be regretted：不是系表结构，而是be to do结构，表示“注定做某事；不可避免做某事”。

【问题讨论】两种不同性质的“be +不定式”结构

以下两组句子，be之后都是动词不定式，请问它们有何区别？

第一组：

Her job is to look after the children. 她的工作是照顾这些孩子。

My aim is to start up my own company. 我的目标是开办一家我自己的公司。

His ambition was to meet his favourite pop star face to face. 他的心愿是，要面对面地见到他所喜爱的流行歌星。

第二组：

He is to stay here until we return. 在我们回来之前他必须呆在这里。

Such people are to be pitied. 这种人应受到怜悯。

If we are to succeed, we must rely on the masses. 我们要成功必须依靠群众。

英语中，“不定式作表语”和表将来的“be +不定式”结构，是两个不同的概念。第一组是“不定式作表语”； 第一组是表将来的“be +不定式”结构。我们用最简单的方法做一区别，请看这两个句子：

Her job is to look after the children. 她的工作是照料这些孩子。

She is to look after the children. 她将要照料这些孩子。（表将来，可能是安排的任务）

这两句话只是主语不同，其他部分看起来完全一样，意思却大不相同，这是为什么？

它们看起来太相似了，以至于有时难以区别，这确实是一个很有意思的问题。

Her job is to look after the children. 意为 “她的工作是照料孩子的。” 本句中，is 是系动词，to look after children是不定式作表语。主语和表语可以互换而意思保持不变：To look after the children is her job. (照料孩子是她的工作。)

She is to look after the children. 意为“她将要照料孩子。”本句中，is是助动词，跟不定式to look一起，表示将来。因为不是主系表关系，所以is前后两部分不能颠倒，不可以说：To look after the children is she.

问题中的第一组都可以“主表颠倒”，变成：

To look after the children is her work. 照顾这些孩子是她的工作。

To start up my own company is my aim. 开办一家我自己的公司是我的目标。

To meet his favourite pop star face to face was her ambition. 面对面地见到他所喜爱的流行歌星是他的心愿。

第二组则不可以颠倒，因为颠倒后句意逻辑不成立。

【判断规则】A代表“主语”，B代表“不定式”：

“主系表结构”：A is B. 可以说：B is A. （is意为“是”）

“将来时结构”：A is B. 不可以说：B is A. （is to do意为“将要做”）

▲关于不定式表将来的深层分析：

该问题一般语法书都有论述，但大多不够详细。下面详细解说：

在不同的语境中，“be+动词不定式”结构可以表示多种不同的语义。

一、表示“计划”或“安排”。

This line is to be opened to traffic next week. 该线路将在下周通车。

They are to meet at the gate of the school. 他们将在学校门口见面。

二、表示“命令”。一般译为“必须”或“不准”。

No one is to leave the room without permission.

This naughty boy is to stay here until class is over.

三、表示“可能性”。

Are you to pay the debt for your wife?

The lovely puppy is nowhere to be found.

四、表示“意图”或“打算”。一般译为“想要”。

If you are to be there on time, you had better hurry.

If all of us are to remain friendly, we must be sincere with one another.

五、表示“应该”。

You have done so well. You are to be rewarded.

Such nasty questions are to be avoided.

六、表示“不可避免”或“注定发生”，意为“必定”。例如：

Your plan is to be a failure.

The murderer was sentenced to life imprisonment and was never to see his family again.

She did not think too much of her unkind acts. However, they were to be harmful to herself, too.

Tom wasn't worried about his head injury, but it was to be troublesome for him later.

七、表示“期待”。

The actual use of the newly invented equipment is still to be determined.

Henry's appointment with the firm is yet to be confirmed.

八、表示与将来不符或相反的“假设”。

If it were to rain tomorrow, we would not go outing.

Even if you were to take a taxi now, you would not be able to arrive at your destination on time.

▲关于不定式作表语的深层分析：

该结构说明“不定式是主语的内容或解释”。这种结构的实质是：be是系动词，be前的主语和be后的表语是同位概念，不定式是主语的内容和解释，主语往往表示“目的，目标，想法，愿望，爱好，工作，责任，方法”之类的名词。

因为它们是“主语和表语”的关系，是对等的关系，所以，主语和不定式的位置可以颠倒，而意思保持不变，例如：

A salesman's job is to seek out customers.  推销员的工作是寻求顾客。

Her purpose in learning is to be a doctor. 她学习的目的是当医生。

My only hope is to use her notes.  我唯一的希望是用她的笔记。

His dream is to become an engineer. 他的梦想是将来成为一名工程师。

The trick is to hold your breath while you aim.  窍门就在于瞄准时须屏住呼吸。

Our goal is to achieve the maximum of efficiency.  我们的目标是取得最高的效率。

The purpose of a trap is to catch and hold animals. 陷阱是用来捕捉野兽的。

His target is to save $100 a week. 他的目标是每周储蓄一百元。

The only way to do that is to reduce expenses.  做到这一点的唯一办法是减少开支。

His sole motive is to make more money. 他唯一的动机就是多赚钱。

Our only resort is to inform the police.  我们唯一的办法就是向警方报案。

His hobby is to observe things with his telescope.  他的嗜好就是用望远镜观察东西。

His ambition is to fly to the moon one day. 他的愿望是有一天登上月球。

Our aim is to make books available to all. 我们的目的是使大家都能看得到书。

My chief end is to serve my country and my people.  我的首要目标是为国为民服务。

Her wish is to go abroad for advanced study. 她的心愿是到国外进修。

One of my duties is to file the letters.  我的职责之一是将信件归档。

Our thought is to avoid war. 我们的意向是避免战争。

My idea is to climb the mountain from the north. 我的想法是从北面登上这座山。

What Tom wants is to do his field work.  汤姆想的是作他的野外实习。

The new manager's job is to restore the company to profitability.  新经理的工作是让这家公司能恢复盈利状况。

Her work is to look after the children. 她的工作是照顾这些孩子。

My aim is to start up my own company. 我的目标是开办一家我自己的公司。

His ambition was to meet his favourite pop star face to face. 他的心愿是，要面对面地见到他所喜爱的流行歌星。