Question

2) But that fact did not lessen the glow of pleasure which so good-natured an action had given me.
(Make affirmative sentence)​

Answer 1

But that fact failed to lessen the glow of pleasure which so good-natured an act had given me.

=> Here, we have replaced "did not lessen" with "failed to lessen".

=> On removing "not" the sentence becomes affirmative.