特殊疑问词 wh-word + do ...，是由“特殊疑问句”转化而来，作用相当于一个名词，在本句中作宾语。特殊疑问词始终在前，不可能在谓语动词之后。例如：

I don't know what I should do next.（正）

I don't know what to do next.（正）

I don't know to do what next.（误）

同理：

I can't decide which one I should buy now.（正）

I can't decide which one to buy now. （正）

I can't decide to buy which one now. （误）

【说明】

（1）这种转换在英语中很常见。

（2）注意术语的不同：

What should I do next? —— 叫做“直接疑问句”。

I don't know what I should do next.——叫做“间接疑问句”或“名词性从句”。

Which dress do you like best, madam?

Sorry, I can't decide which one to buy now.

A. to buy which one  

B. which one to buy 

C. buy which one 

D. which I should buy it

I can't decide which one to buy now(不定式短语作宾语）

=I can't decidewhich one I will buy now（宾语从句作宾语）

句子语法结构分析如下

主语：I

谓语：can't remember

宾语：which one to buy

时间状语：now

解答如下：

以上结构为：动词+疑问词+to do结构，疑问词+to do可以看作为不定式短语作前面动词宾语，也可以看作为一个疑问词引导宾语从句作前面动词宾语的简化形式。

(1)疑问词+不定式to do 作为不定式短语

We must know how to operate the machine. 

我们必须弄懂如何操作这机器。

解析：疑问词how作不定式逻辑方式状语。

(2)疑问词+不定式to do 作为从句简化理解

We must know how to operate the machine. 

=We must know how we will operate the machine.

我们必须弄懂如何操作这机器。

解析：以上疑问词+不定式可以还原为一个从句作动词宾语。

备注(1)：疑问副词+不定式，疑问副词作不定式逻辑状语

备注(2)：疑问代词+不定式，疑问代词作不定式逻辑主语，逻辑宾语，逻辑表语。

可以接疑问词+不定式动结果作宾语的及物动词有如下：

【这样动词有：decide决定, know知道, consider考虑 forget忘记, learn学习, remember记得, show展示, understand明白,  wonder想知道, hear听说, find out查明, explain解释, tell告诉；discuss讨论 ,teach教，】“疑问词＋动词不定式”作宾语时，相当于一个由该疑问词引导的宾语从句。因为“特殊疑问词＋动词不定式”这种结构相当于一个名词性从句，所以可用同等成分的从句代替，从而把简单句转换为复合句。改写时只需在疑问词后加上适当的主语，并把不定式改为适当的形式的谓语即可。

A:动词+特殊疑问词+to do结构

We must know how to operate the machine. 

=We must know how we will operate the machine.

我们必须弄懂如何操作这机器。 

Do you know when to start? 

你知道什么时候开始吗？ 

I showed her which button to press.

我告诉她应该按哪一个按钮。 

I know where to find the boy.

我知道到哪里能找到这个男孩。（作动词know的宾语）

He has to learn how to collect water, hunt for food, and make fire.

他得学会收集淡水、寻找食物，生火什么的。

He knows how to play the piano.

他会弹钢琴。

I don’t know what I will do next.

=I don’t know what to do next.

我不知道接下来该怎么办。

Can you tell me how I can improve my English?

=Can you tell me how to improve my English? 

你能告诉我怎样提高我的英语吗？

The doctor told him when he should take the medicine.

The doctor told him when to take the medicine.

医生告诉他什么时候吃药

We can't decide when to start.

We can't decide when we should start.

我们没有决定什么时候出发。
I can’t decide which to choose. 

我不能决定选哪一个好。

Did you ask her which to buy? 

你问没问她该买哪一个？