The research mainly deals with the difficulty the students h

the students have是前面省略了that的定语从句,修饰the difficulty 的,去掉不看,就是：
The research mainly deals with the difficulty focusing attention on their study all thetime
difficulty和focus是主动关系,用现在分词
这份研究主要是解决学生们在学习中（能否）专心的困难