What Yang Liwei wanted to do when he got out of the spaceship was (to) share the joy with all the Chinese. 

本题考查的“语法点”有两个：（1）系表结构（不定式作表语）；（2）动词不定式省略to。

分别讲解如下：

（1）不定式作表语，说明“不定式是主语的内容或对主语内容的解释”。

这时，主语和不定式的位置可以交换，即：主语和表语可以调换位置，而意思保持不变。例如，网友的句子：

What Yang Liwei wanted to do when he got out of the spaceship was to share the joy with all the Chinese.  等于：

To share the joy with all the Chinese was what Yang Liwei wanted to do when he got out of the spaceship.

该句的主语是一个名词从句What sb. wanted to do，相当于一个名词。通常情况下，使用名词居多。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。

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，方法等”之类的名词，或者是某人想做的事情（如本句）等。

（2）关于不定式省略to 的问题：

 在下列情况下，作表语的不定式可以省略to，例如：

 What he wanted to do was (to) become a skillful worker.

他的愿望是成为一个技术熟练的工人。

All I want to do now is (to) drink some water. 我想做的一切就是喝点水。

The first thing for the thief to do is (to) run away. 小偷想做的第一件事就是逃跑。

以上例句中，主语部分含有行为动词do，而且往往含有all, the first 等表示“所有，唯一”等概念的词语，或what 引导的主语从句。当然在这种情况下，to不省略也好。

【注意】本句的主语是个名词性从句，表达一个“事件”。如果选用 was about to share，那么就成了将来时，这时的主语应该是“人”，而不是“事”。所以，讲不通。