Learn Russian: Этот огромный камень нельзя сдвинуть.

When нельзя is followed by a perfective verb, it means the action is physically impossible to achieve or will not succeed. The sentence means that you physically lack the ability to move this stone. If we used the imperfective aspect instead, the meaning would completely change to a rule or prohibition!

What would the sentence mean if we used the imperfective verb сдвигать instead?

Этот огромный камень нельзя сдвигать would mean that it is strictly forbidden to move this stone (perhaps it is a historical monument or part of a booby trap). With нельзя, the imperfective aspect always indicates a prohibition rather than an impossibility.

Is Этот огромный камень the subject of the sentence, meaning it is in the nominative case?

It looks exactly like the nominative, but it is actually in the accusative case! It is the direct object receiving the action of the verb сдвинуть (to move what? the stone). Because камень is an inanimate masculine noun, its accusative form is identical to its nominative form.

What does the prefix с- in сдвинуть tell us about the movement?

The base verb двинуть simply means to move or give a push. Adding the prefix с- specifically means to shift something off its current spot or dislodge it. So сдвинуть means to successfully shift that huge stone away from where it is currently resting.