[not before noun] confident that you know something or that you are right ‘Is that John over there?’ ‘I'm not sure’. You don't sound very sure. sure (that)… I’m pretty sure (that) he’ll agree. Are you sure you don't mind? I’m sure he’s still alive. sure of something I hope you are sure of your facts. sure about something Are you sure about that? sure how, whether, etc… Ask me if you're not sure how to do it. I'm not sure whether I should tell you this.