Answer:
To change Affirmative sentences to negative sentences, we just have to add "not".
![\rule[22]{225}{2}](https://img.qammunity.org/2021/formulas/english/high-school/hg8ns2wbjhswirln9563w1fkavazc5dech.png)
So, The sentences will be :
1) Mary is not prettier than Stella.
2) They have not unaccepted the invitation for the party.
3) A Jackal is not cleverer than a Fox.
Hope this helped!
~AnonymousHelper1807