- 老师，为什么后面was后面跟那个动词原形sing啊？

- 这里本来应该是用 to +动词原形的，比如

在 The Horse and His Boy 这本书里面的这个句子，就是这种结构的完全形式。

Bree said the first thing now to do was to fix a place where they would all promise to meet on the far side of Tashbaan even if, by any ill luck, they got separated in passing the city.

但是，现代英文中，通常喜欢省略中间的这个 to，而在 be 动词后面直接加动词原形。

比如下面这些 Harry Potter 里的句子：
Hagrid was so huge that he parted the crowd easily; all Harry had to do was keep close behind him.

Filch must have hurried off to look for them somewhere else because they didn’t see him anywhere, but they hardly cared – all they wanted to do was put as much space as possible between them and that monster.

再比如 Percy Jackson 中的句子：
I said all he had to do was hide the items away for a while and watch the others fight.

I had won. All I had to do was stab – one quick strike.