Asking for Certainty:
Are you sure?
Are you sure about it?
Are you certain about it?
Do you think it is true?
Do you think so?
How sure are you?
Expressing Certainty:
Yes, I am certain.
I’m a hundred percent certain …
I’m absolutely sure.
I have no doubt about it.
I’m sure about it.
I don’t think there can be any doubt about ….
Of course.
I’m positive.
I’m quite sure about it.
I’m no doubt about it.
I’m absolutely certain that…
Expressing Uncertainty:
I’m not sure about it.
I doubt it.
I’m not really sure about…
I don’t know for sure…
It’s very unlikely.
I have my own doubts.
I don’t think so.
I don’t believe this is true.
There’s some doubt in my mind that …
I’m not a hundred percent sure.
I don’t know yet.