确实成分不同.think it hard to solve that problem 中it是think的宾语（形式宾语,代替后面的to solve that problem）,hard是宾语补足语.在think it's hard中,think后面是个结构完整的宾语从句,it是此从句的形式主语,hard是此从句的表语,it代替后面的to solve the problem.祝好!
补充：
我又确认了一下 应该是都可以的 下面是think it adj to do 的例句
think it necessary/possible/best etc
(=believe it is necessary,possible etc)
I thought it best to call first.
I thought it appropriate to invite her to speak at the meeting.