doubt的肯定形式后接whether/if从句,否定和疑问形式后接that从句
I doubt whether/if he can speak English.(肯定句)
I do not doubt that he can speak English.（否定句）
Do you doubt that he can speak English?（疑问句）
而be sure的肯定疑问形式后接that从句,否定形式后接whrther/if形式
I am sure that he can do it well.(肯定句)
Are you sure that he is a teacher?(疑问句)
I am not sure whether/if the news is true.(否定句)