All you need to do is (to) fill in this form.中to为什么可有可无?是什么语法现象?

前面有to了后面的可以省略
动词不定式是由“to+动词原形”构成的.在实际运用中,小品词to在某些场合会被省略,下面简要归纳一下这些特殊情况.
1．在某些表示感官的动词（组）如fell,listen to,hear,look at,see,watch,notic等的复合宾语中,动词不定式不带to.例如：
We often hear her sing this song.
我们经常听到她唱这首歌.
I saw him play basketball.
我见过他打篮球.
2．在使役动词make,let,have等的复合宾语中,动词不定式不带to.例如：
Now let me hear you play.
现在让我听你演奏.
They made him work all day.
他们让他整天工作.
注意：上述两类句子若是变为被动语态,则原来作宾语补足语的不定式须带to.例如：
She is often heard to sing this song.
He was made to work all day.
3．动词help的复合宾语中,动词不定式既可带to,也可不带to.例如：
Will you help me learn English?=Will you help me to learn English?
4．help后面的动词不定式作宾语时也可省去to.例如：
Then he helped sweep the floor.
那时他帮助扫地.
The new policy has helped raise his wages.
新政策使他提高了工资.
5．当两个或两个以上不定式短语连用而且作用相同时,那么除第一个不定式短语用to外,其余的不定式短语可省略to.例如：
This article is easy to read and understand.
这篇文章浅显易懂.
He taught us to read and write.
他教我们读书、写字.
但是,若是在对照的场合（即强调对比）时,则不可省去to.例如：
It is better to laugh than to cry.
笑比哭好.13:26 2009-5-10ryedu.net
6．在had better,would rather,why not…固定结构中,动词不定式不带to.例如：
You’d better go home now.
你现在最好回家.
She would rather stay here.
她宁愿呆在这儿.
7.当介词but,except,besides前面有一个实义动词do时,不定式在介词之后可省去to.例如：
We can do nothing but wait.
除了等待之外,我们别无办法.
What do you like to do besides play piano?
除了弹钢琴之外,你还喜欢做什么?
8．如果不定式作表语,且主语部分有all,what,only或是*形容词修饰的名词和实义动词do时,作表语的不定式可省去to.例如：
All you need to do now is have a good rest.
现在你所需要的是好好休息.
What I want to do is finish my homework.
我要做的是完成作业.
The best way to do is wait till she returns.
最好的办法是等她回来.
The only thing you can do is sleep.
你惟一可做的只有睡觉了.