In English, I would probably say "...but I doubt it."
Or "but I have my doubts", in the UK at least.
In the US as well.
Duo doesn't accept it, though.
This topic has so many mistakes it's unbelievable. 'un doute' is not plural and should be 'but I doubt it'.
The French idiom uses a singular noun; one English equivalent uses the plural, and another uses a verb instead of a noun. Those aren't 'mistakes'.
Hmmm.. The English idiomatic use of plural doubts in this context would usually be " but I have MY doubts".
Is doubts always singular in French or could one say "des doutes" here
Duo doesn't accept 'I'm doubtful'.
Wise doubts - if she were a queen, she'd have a flunky to answer the phone.