- I doubt that she will pass this year.
- I doubt whether she will pass this year.
Which one is the correct sentence?
There are two possible outcomes: she will pass, or she will fail.
When you use the “I doubt that…” construction, you are saying that you think it more likely she will fail.
When you use the “I doubt whether…” construction, you are saying ‘She might pass, but I also think there is a possibility she will fail’.
[color=indigo]I doubt that he will come to your party: it’s more likely he won’t come.
[color=indigo]I doubt whether he will come to your party: I don’t think he will come, but I could just as easily be wrong, and he does come.