我了解当主语是 all / what...do 时，后面作表语的不定式都可以省略 to。如：
All / What I did is close the door.
All / What I want to do is go to the countryside.
但若是下面样的句子呢？
The first thing to do is _______ him the news.
这里也可以只填 tell 吗？
可以。The first thing to do is tell him the news.
英语句子在下列状况下，作表语的不定式可以省略to：
主语部分含有实意动词do（包含do 的其他形式或非谓语形式），同时含有all（所有定义）, the first（序数定义）, the best（最高级定义）等表示“所有，唯一”等定义的词汇，或由what 引导的主语从句。比如：
All I want to do now is drink some water. 我想做的所有就是喝点水。
All she did yesterday was sweep the floor. 她昨天做的所有就是打扫了地板。
The first thing for the thief to do is run away. 小偷想做的第一件事就是逃跑。
If you want further information the best thing to do is have a word with the driver as you get on the bus.假如你想知道更多信息,最好的方法是在上公交车时问一问司机。
（来自柯林斯例句）
What he wanted to do was become a skillful worker.他的愿望是成为一个技术熟练的工人。
当然，在上述状况下，to不省略也好。
