I still doubt whether the practical value of this project really exists.

我仍然怀疑这个项目是否有实际价值。

I doubt whether Alyssa will accept the invitation the day after tomorrow.

我怀疑艾莉莎是否会在后天接受邀请。

He is so closely hemmed in on all sides that I doubt whether he could escape even if he grew a pair of wings.

密密层层的天罗地网，我看他插翅难飞。

I doubt whether he is still alive.

我怀疑他是否还活着。

I doubt whether he will come.

我怀疑他是否会来。