What a kid needs to do is tell others 中的is后的to为何可以省略

解答：假如当主语部分有动词do的某种形式时，用作表语的不定式可以省略to。

What you have to do is fill in the form. 你要做的是填好这份表格。

All I did was press the button. 我只不过按了一下按钮。