Translation:The witch could transform the alien into a human.
Note that while English uses "could" for both the subjunctive and past tense of "can", the two forms are different in German. Would someone like to test themselves explaining which form this is, and what the other would be? :)
I am so struggling with some of this. I'll go for 'subjunctive' in answer to your question as it expresses the possibility of doing something rather than something having been done. Could you tell me why I can't say "The witch would be able to transform the alien into a human"? It isn't accepted, but I'm struggling to understand why it's different in mean from 'could'