楼主,that 不能与语句用逗号隔开,不管做目的还是结果状语,这是对的.但so that 后用不用情态动词,要根据句子实际情况而定,例如：
The term and condition are printed in very small letter so that they are difficult to read.
条款与条件是用很小的字体印刷的,以致难以阅读.
You may flour the pastry board so that the dough doesn't stick to it.
你可以在做糕点的面板上撒些面粉,这样揉好的面粉就不会粘到板上了.