有一读者问： 我们知道so that从句可用might和could，如：
(1)He gave her the cup so that she might drink （或could drink）.
但请问其否定结构是否也是一样？
你所举的so that从句是表过去的目的的状语从句，其中除可用情态动词might和could外，还可用should和would。再如：
(2)I arrived early so that I should get a good view of the procession. （亦可用could, might或would）
如用表过去的so that目的状语从句的否定结构，则只可用情态动词might, should或would, 但不可用could，如：
(3)I arrived early so that I might （或should 或would ）not miss anything. （不可用could not）
更为自然的说法是用so as not to，如：
(4)I arrived early so as not to miss anything.
但表过去的so that引导的结果状语从句却可用could not，如：
(5)I chained the dog up carefully, so that he couldn't get away. （也有人将这里的so that唤作等立连词，引导一个分句）
