Hi, I'm Italian. "Niente" can be both the subject and the object, it depends from the interpretation:
1) Most common and logical for an Italian, because it is a common saying: "Non cambiava niente" = "Niente cambiava" (Nothing changed) = Niente is the subject.
2) It could also be: "Non cambiava niente" = "Lui/lei non cambiava niente" (He/she changed nothing) = Niente is the object. But this case needs a context.
So what is the object? Who/What is doing the changing? I said "It was changing nothing". How would you say that?
In the same way. You are right, you can interpret the sentence with "niente" as the object. "cambiare" can be an intransitive or transitive verb: in the first case you translate "nothing was changing" (niente is the subject and the meaning is of the sentence is, more or less, "change wasn't happening"), in the latter "He/she/it was changing nothing" (niente is the object).
Una delle traduzioni è 'it didn't change anything'. Allora perché si usa l'imperfetto ?
No, present perfect doesn't work in this case. The sentence is about a continuous action happening in the past.
Why can't I use "would" in place of "did" in this sentence? I wrote "It would not change anything." And it underlined "would" as incorrect :/
I think "it would change nothing" which is what i wrote - as in when you are reporting about an event and your statement is that nothing was changing - should be accepted. Just as we say i was playing in the yard or I would play in the yard or I did play in the yard - all acceptable forms of the imperfect.