33. Did Peter's parents be happy if Peter got good marks? 

34. His grandparents have been visited.

35. If we had enough money, we would go on holiday last year.

36. Unless he works hard, he'll fail the test.

37. Dolphins are not fish. They are a mammal that lives in water.

38. Dolphins are the most intelligent animal on Earth.

39. Though dolphins can be found in all oceans in the world, they prefer coastal waters and bays.

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33. Did Peter's parents feel happy if he got good marks? 

Were Peter to get good marks, his parents would be happy. ( Câu này viết lại câu với Were thì hợp lí hơn vì đó là trường hợp đảo ngữ của câu điều kiện loại 2.)
34. His grandparents have been visited by him.
35. If we had had enough money, we would have gone on holiday.
36. Unless he works hard, he'll fail the test.
37. Dolphins are not fish. They are mammals that live in the water.
38. Dolphins are one of the most intelligent animals on the Earth.
39. Though they can be found in all oceans in the world, dolphins prefer coastal waters and bays.