1. I'm absolutely sure. 2. I'm positive that... 3. I have no doubt that... 4. I'm a hundred percent certain. 5. I'm convinced that... 6. Chances are that... (= this will probably happen) 7. Odds are that... (= this will probably happen) 8. I seriously doubt it. 9. I don't think so. 10. Probably not. 11. It's not very likely. 12. There's not much chance of that. 13. I'd be very surprised if that happened. 14. I wouldn't bet on it. (= there's a small chance it could happen... but it probably won't happen) 15. That'll never happen.