There is a difference between the reference to a context and an unknown subject.
a) This is too much for me.
b) It rains.
c) It was just impossible.
In phrase a) and c) the reference is made to a context. We see this can happen with this/that or also with it.
In phrase b) there is just the subject unknown. Here we cannot use this/that, but only it.
This is impossible. => It is impossible. It rains.
=> That rains.
In French only one option is correct. With il the unknown subject is referred to. The context is referred to with ça