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Here are the answers: 1. If I see him, I will give him a gift. 2. If I had a typewriter, I would type it myself. 3. If I had known that you were in hospital, I would have visited you. 4. You could make better progress if you attended class regularly. 5. If I knew his telephone number, I'd give it to you. 6. If you had arrived ten minutes earlier, you would have got a seat. 7. He could get a job easily if he had a degree. 8. I shouldn't have believed it if I hadn't seen it with my own eyes. 9. If he studies hard, he can pass the exam. 10. If she had asked you, would you have accepted? 11. Rice plants grow well if there is enough rain. 12. He might get fat if he stopped smoking. 13. What would you do if you found a burglar in your house? 14. If she didn't hurry, she may be late. 15. If I had tried again, I think that I would have succeeded. 16. Water does not run downhill if there were no gravity (or) Water doesn't run downhill if there is no gravity. 17. If I tell you a secret, will you promise not to tell it to anyone else? 18. Tell him to ring me if you see him. 19. The police will arrest him if they catch him. 20. If you had spoken more slowly, he might have understood you. Here are the rewritten sentences using "unless" instead of "if": 1. You will cut yourself with that knife unless you are careful. 2. I'll bring you another unless you like this one. 3. I won't be able to do any work unless I have a quiet room. 4. She'll be late unless she hurries. 5. Our crops would be better if we had more rain. (This sentence cannot be rewritten with "unless" without changing the meaning. "Unless" implies a negative condition, but in this sentence, more rain is a positive condition.) 6. The crops would have been ruined unless the flood had risen no higher. (Rewritten to convey the opposite condition) 7. He wouldn't have known that we were there unless you had sneezed. (Rewritten to convey the opposite condition) 8. She would look quite nice unless she does her hair in the current way. (Rewritten to convey the opposite condition) 9. We would not have lost the game unless John had not played for our football team. (Rewritten to convey the opposite condition) 10. I wouldn't give up my job unless I won a big prize in a lottery. (Rewritten to convey the opposite condition) Note: Sentences 5, 6, 7, 8, 9, and 10 were rewritten with the opposite condition to maintain the original meaning, as "unless" typically implies a negative condition.