具体参考如下：\r\n（1）连词连接两不定式前面的有do后面的to do的to省略，如： \r\n I have nothing to do but watch TV all day long. \r\n Tom have no choice but to give it up.\r\n（2）用is连接，不定式作表语时前面主语有do时后面表语to do的to省略，如：\r\n W...

是可以省略“to”在不定式作表语,主语部分又有all,what only,或有最高级形容词修饰名词和实义动词“do”时，"to"可以省略

（1）连词连接两不定式前面的有do后面的to do的to省略；如： I have nothing to do but watch TV all day long. Tom have no choice but to give it up.（2）用is连接，不定式作表语时前面主语有do时后面表语to do的to省略；如： What I want to do is stay at home. What I want is...

but和except后。but前是实义动词do时，后面出现的不定式不带to。 如：He wants to do nothing but go out. 他只想出去玩。不定式做表语时，一般要带to,但若主语部分中含有do的某种形式时，符号to可省去。 如：we’ve missed the last bus. All we could do now is walk home.

主语带do表语省略to的情况 当主语部分有动词do的某种形式时，用作表语的不定式可以省略to。如：All you do now is (to) complete the form. 你现在要做的只是把这张表填好。The only thing to do now is (to) go on. 前进是现在唯一的出路。What I’ll do is (to) tell her the truth....

动词不定式 to 的省略 1.主语部分有 to do ,系动词 is 或 was 时 ,作表语的不定式通常省去 to。如:The only thing you have to do is press the button.你必须做的惟一事情是按按钮。2.作介词 but ,expect ,besides 的宾语,前面又有实意动词 do 时,不定式通常省去 to. 如:He said that...

具体参考：（1）连词连接两定式前面do面to doto省略： I have nothing to do but watch TV all day long. Tom have no choice but to give it up.（2）用is连接定式作表语前面主语do面表语to doto省略： What I want to do is stay at home. What I want is to stay with you.（3）...

这个主语里的do是助动词，也就是do这个单词本身，并不是指其它实义动词的原形

can do things，只是因为things是主语 ，he can do是that引导的一个从句，因此do跟wait之间没有固定搭配一说，这句话的主句是The only thing is （to）wait.wait作动词时The only thing is to wait. wait也能做名词，The only thing is wait.也通。do如果后面加动词，一定需要一个介词to。

后面的表语选择动词不定式，不使用现在分词，所以不能用你问的doing，即：The first thing to do is (to do sth.)在上述结构中，由于主语The first thing to do中含有一个实意动词do，因此表语中动词不定式要省略to，这就形成了The first thing to do is buy the required books.这个句子。