Question

He had not eaten since the previous nightfall (make it affirmative)

Answer 1

I HAD EATEN SINCE THE PREVIOUS NIGHTFALL

Answer 2

He had eaten since the previous nightfall.

This is the right affirmative sentence and it could be changed according to the subject and verb.

This totally lies in the correct sentence and position can be happen due to the previous nightfall considered for affirmative sentence.

This is overall named with affirmative sentence and had eaten since the previous nightfall.