"Nie myślę, że ten pomysł jest gorszy."

Translation: I do not think that this idea is worse.

[Jack.Elliot]

i am not thinking that this idea is worse

[BronwynYoung]

Could someone please tell me why we don't use genitive here? (Negation)

[Arnoldpitt]

"That" is more often omitted in English

[Jellei]

Possibly, but it's safer to have some equivalent of "że" in the main translation. You may of course omit it, it will be accepted.

[JKWolf17]

I dont think that this idea is the worst

[Jellei]

"the worst" is "najgorszy", not "gorszy".

[steve571213]

'I do not think that this is a worse idea' should be accepted?

[Jellei]

As usual, we're strict about the 'not changing the subject of the sentence' thing. Well, in this case, 'of the clause'. You changed the subject from "this idea" to "this", which in Polish would be rendered as "Nie myślę, że to jest gorszy pomysł".