当主语部分有动词“do”且作表语的不定式又是说明“do”的具体内容时，不定式符号“to”可有可无，但要注意两点：

1.系动词be是一般现在时或一般过去，不定式作表语可加或不加to。例如：

What he should do is / was (to) apologize to her. 他应该做的就是向她道歉。

2.若系动词be是其他时态，则不定式作表语要加to。例如：

The teacher said that all we should do would be to give the classroom a thorough cleaning. 老师说，我们该做的就是将教室彻底打扫一下。

曾老师提到如果谓语为will/would be时，作表语的不定式不可省略to. 我以前没有看到过这种说法。曾老师可否告知这个说法出自哪里。

我查了COCA语料库，would be后用不带to的不定式的为12/200=6%.

is和was后用to do的都是大约8%。

也就是说，主语部分含有实意动词do, 不定式作表语，如果系词be为is 或was, 则绝大多数情况下不定式不带to, 但大约有8%的几率，不定式是带to的。

当系词为will be时，不定式作表语较少，如果用不定式作表语，则通常都是带to的。

当系词为would be时，不定式作表语通常都带to, 但有6%的几率，不定式不带to。也就是说不是绝对不能用不带to的不定式作表语。

以下为COCA例句：

...all I'd want to do would be sleep.

...one of the best things that Obama could do would be build 12-month farmers markets, especially in inner cities.

 All she would have to do would be not get caught until that happened. 

 ...the very first thing you would do would be interview the parents.

 I thought the very least that they would do would be whip me because they whipped people on a regular basis.

 We thought that the best thing to do would be bring in the bloodhounds.

The most graceful thing to do would be just not be there, and that worked out all right. 

Last thing she'd want me to do would be share her nail polish.

 As governor, what I would do would be work very closely with the president and with the Congress to ensure that...

 The best thing to do would be check her in about a year and see how she did. 

All we'd have to do would be bait a trap with fruits or nuts.

...the first thing I would do would be pay off my debts...

    1. 如果主语部分含有主动词 do (不是助动词 do)的某种形式（谓语动词或者非谓语动词均可），作表语的动词不定式通常不带 to ，也可以带 to 。（很容易在词典中找到例句）

    2. 有些语法著作在肯定了上述结构通常省略 to 的同时还指出，在由 what 等结构引导的主语部分中，如果谓语动词部分的 do 是完成体形式，后面的补语（即表语）也可以用-ed 分词分句表示，如果 do 是进行体形式，则补语必须用 -ing 分词分句，举例如下：

    What he'll do is spoil the whole thing.

    What he's done is spoil the whole thing.

    What he's done is to spoil the whole thing.

    What he's done is spoilt the whole thing.

    What he's doing is  spoiling the whole thing.

    （摘自章振邦主编《新编高级英语语法》p.525）

句子主语内容如果有do的形式，作表语的不定式可以省略前面的“不定式符号to”

The last thing we wanted to do is (to省略了)cause her any more hardship and emotional turmoil.

参考译文：我们最不想做的事就是给她带来更多的困难和情绪上的混乱。

What you need to do is (to可省略) give your suit a quick brush.  

你只需要把你的西装很快地刷一刷就行了.

What you need to do is (to可省略) perform loudly, clearly, fluently, and vividly.  

你们只需大声清晰流畅生动地演出你们的风采.

What you need to do is (to可省略) tell the truth.  

你要做的就是讲真话.