kick和have不是动词原型，而是不带to的不定式。当不定式作表语时，主语部分含有实意动词do, 则表语不定式的to通常省略，但不省略也是可以的。例如：

What he will do is (to) sell his house.

What he did yesterday was (to) sell his house.

What he wants to do is (to) sell his house.

在实际应用中，不带to的结构更为常见。

这种结构语法书称为假拟分裂句，强调句子的实意动词。如果不强调，以上三句则分别为：

He will sell his house.

He sold his house yesterday.

He wants to sell his house.

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. 若系动词be是一般现在时或一般过去时，不定式作表语大都不加to，加to的情况较少。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，则不定式作表语大都是加to的，不加to的情况较少。

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。

这是中学里传说中的所谓“有do无to”问题

cleft-sentences 指英语中的分裂句（我们常说的强调句）

除了but后的情况，“有do无to”

主要是all-cleft与what-cleft中（这两种假拟分裂句pseudo-cleft sentences）

我想题主的意思，是为什么这种do后可以不要to,

先从简单的说起

He wanted to do A and to do B.

也可说成 He wanted to do A and do B.

假使后面一个动词需要to, 但在前面的结构中，已经暗含了这个to, 你就可以省略

（这只是一个比方，假拟分裂句中的情况实际并非如此）

All-cleft:

All you had to do was turn on the computer.

这两个划线的部分，是平行结构。除了结构的平行，

还有语义的平行。这个do是什么含义？是 turn on the computer.

all-cleft的焦点信息就是，你只做了一件事，没有第二件。

所以，

现在的问题不是为什么不要to, 而是为什么要那个to：

表语就是主语补语，两者可以互换位置，

To turn on the computer was all you had to do

这个to 得加.

What-cleft

1-To talk to him is what I did.
2-Talking to him is what I did.

3-What I did was talk to him.
4-What I did was to talk to him.
5-What I did was talking to him.

1-2都对，

3-5里面，5语法也没什么错，但罕用。

what-主语从句与其补语互换位置，很容易理解

 考察句子3

3-What I did was talk to him.

a. 纯对称的情况：

What I did was I talked to him

这句话的确是有人用的，但，是不规范的

实际它来自：

What I did was, I talked to him.的某种缩合

b. 关于to的问题

What I did was talk to him.

= All that I did was talk to him.

= All I did was talk to him.
回到了all-cleft结构。

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结论是，两种假拟分裂句，牛人一般都不用to, 用to很难说它是错的结构

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其它例子选：

All I could do was (to) try to...

All I had done was (to) try to ...
All I am doing is trying to ...

All I was doing was trying ...

All I was doing was washing sth

What I did was (to) shoot some birds …

    如果句子的主语部分含有动词do 的某种形式（谓语动词和非谓语动词 的do都可以），作表语的不定式可以省略to,也可以不省略。

    如果句子的主语部分没有动词do 的某种形式时（谓语动词和非谓语动词 的do），作表语的不定式必须带to。