今天看到一道题目，原题是如此的：
It's not complicated at all.______ you need is _______ the button and the machine will do the rest.
答案是It's not complicated at all.What you need to do is press the button and the machine willdo the rest.
我感觉，第一这是个主语从句，主句的动词是is,是个主系表结构。这里不应该用press, 而是应该用to press或者pressing吧？但标准答案给的press,我不了解是我哪儿考不不周，想请教一下老师。
假如主语部分含有 do 的某种形式 ，作表语的动词不定式可以省略 to，也可以保留 to。所以：
It's not complicated at all. _What_ you need is _press_ the button and the machine will do the rest.
It's not complicated at all. What you need to do is press the button and the machine will do the rest. （正确）
It's not complicated at all. What you need to do is to press the button and the machine will do the rest. （正确）
