Here are the rewritten sentences structured with "if" clauses:

35. If you eat too much fast food, you will be overweight.
36. If we try to recycle things, we will save natural resources.
37. If you finish your homework, you can go out with your friend.
38. If you keep your feet warm, you cannot catch a bad cold.
39. If you study hard, you will pass the exam.
40. If you are careful, you won’t break that valuable vase.
41. If you ask him for advice, you will find a way out.
42. If we keep our environment clean, we will enjoy a healthy life.
43. If you promise to pay back the money, I will lend you some.
44. If you are hard-working, you will succeed.
45. If you have a dictionary, you can look the word up.
46. If you are careful, you won’t cut yourself with that knife.
47. If we save energy, our sources of energy won’t soon end.