"彼の計画がだめにならないことを祈る"
- I pray that his plan does not drop through.
- I hope his plan doesn't drop through.
- Hopefully his plan doesn't drop through.
こういう場合は "fall through" か "fall apart" を使うと思います。
"go to ruin" も使えます。
I'm praying so that his plan doesn't drop through.
I hope his plan succeeds.
「彼の計画がだめにならないことを祈る」は英語で「I'm praying so that his plan doesn't drop through.」と言います。「drop through」はあまり聞いたことがなくてより自然の言い方は「I hope his plan succeeds.」と言います。「だめにならない」というニュアンスを保ちたいなら、「I hope his plan doesn't fail.」が適切です。
彼の計画がだめにならないことを祈るけど、あなたは?
I'm praying so that his plan doesn't drop through. How about you?