Doubt That, Doubt Whether and Doubt If :



If you really doubt that something is true (suspect that it’s false), use “doubt that”:


“I doubt that Fred has really lost 25 pounds.”


If you want to express uncertainty, use “whether”:


“I doubt whether we’ll see the comet if the clouds don’t clear soon.”


“Doubt if” can be substituted for “doubt whether,” though it’s considered somewhat more casual, but don’t use it when you mean “doubt that.”