1）appreciate+it+when（if）+从句这是个固定句型,it是作形式宾语的成分,that不可以作形式宾语（因为形式主语和形式宾语都只能是it）,类似的动词还有like、dislike、enjoy、love、hate、prefer等.举一个例子：
I really hate it if my mom sends me to a school which I don't like at all.
（2）尽管that听起来是对的,但根据语法它是错的.如果非得要用that,可以换成这样：
A：I will let you get on with your job,jerry.
B：I really appreciate that,Tommy.（此时that指代前面那件事）
所以,题目如果要变成appreciate that,就要换成对话的形式：
A：I will do you a fovour, jerry.
B：I really appreciate that, Tommy.
希望您满意!