"He will definitely not forgive you."

Translation: 他一定不会原谅你。

[Kechinsky]

他肯定不会原谅你 should be accepted

[KX3.]

一定、肯定、铁定、注定、必定 should all be accepted.

[bolyglot]

注定 is more like destined , 必定 emphasizes on the inevitability due to causality or the nature of the thing, so I don't think it fits either. 铁定 should be accepted, but it is very colloquial.

[_yuuuuuuu]

all dramas in a nutshell.

jkjk

[SecretAgentNick]

What did I do to not be forgiven? Duo please.