What would help me even more is if you wrote a letter to the parole board telling them I have changed.

这个句子属于口语中的语法错误。注意主句的is，和 I have changed的时态，由此可知what从句和if从句的谓语形式都是虚拟语气，表示与将来事实相反的假设。

我们先把这个句子稍作改动，句子都用陈述语气表示事实。

It really helped me that you wrote to the parole board telling them I had changed.

为了清楚说明假拟分裂句，我们不用外置结构，直接用that从句作主语：

That you wrote to the parole board telling them I had changed really helped me.

划线部分为主语，后接动宾结构。为了突出、强调主句，我们可以用假拟分裂句，即用融合型关系代词what取代that从句：What really helped me, 然后加上系词be, 让that从句作表语，得到下句：

What really helped me was that you wrote to the parole board telling them I had changed.

这样就把原来主句的主谓宾结构，改成了主系表的主句结构，表语为确认性表语。表语位于句末信息中心位置，从而得到强调。

假拟分裂句都是确认性表语，即表语应该是名词性结构。

I need a good rest. 普通句式。

It is a good rest that I need. it分裂句（即强调句型）。

What I need is a good rest. 假拟分裂句，突出a good rest.

A good rest is what I need. 倒装的假拟分裂句。

网友的句子中，What would help me even more作主语，is后应该接名词性结构，但that从句无法表示虚拟条件，于是说话人直接用if从句作了表语。而if从句（条件）是不能作表语的。所以这个句子是语法错误，虽然听话人能明白说话人想要表达什么。正确的表达应该是：

It would help me even more if you wrote a letter to the parole board telling them I have changed.

it指代if从句的内容。为人称代词it的前照应指代。主从句位置可以颠倒：

If you wrote a letter to the parole board telling them I have changed, it would help me even more.

I wasn't sure if you would be back.

if=whether，意为：是否。

表语形容词语义不完整时，通常用介词短语、不定式或that从句补足。现代语法称其为形容词的补足语。wh-从句本可以作介词的宾语，构成介词短语作形容词的补足语。例如：

I wasn't sure (of) when you would be back.

通常认为that从句作形容词补足语时是名词性从句，而wh-从句作为名词性从句受了that从句可以直接位于表语形容词后的影响，介词就被省略了，形成名词性wh-从句直接位于表语形容词后作其补足语的现象。其实wh-从句前加上介词语法都是正确的，只不过现在省略介词更为常见而已。