With the teacher's help, the student got better grades than he would otherwise have done.

1) 书上说这是定语从句。请问这是定语从句，还是比较状语分句？

2) 书上说，关系代词指代先行词grades,充当定语从句的宾语，整个定语从句相当于如下改写的一个完整的虚拟句：

He would not have gotten such good grades if it had not been for the teacher's help.    （if it...help=otherwise改写）

我的疑问是为什么改写的句子出现了一个not, 即He would not中的not, 书上说该句型中，改写than的话，要采用否定not形式，这是我不理解的地方？！

3）书上又说，如果该句型中是that,不是than, 例如：

They support the services that would otherwise be uneconomical.

定语从句可改写为：

The services would be uneconomical if it were not for their support.

The services would be中不出现not, 这是和than上句不同。

换言之，这种句型中，than改写要加not, that改写不要加not. 这是为什么呢？