78. Mary is eighteen years old.(改为同义句) 

Mary is girl.

77. I like singing. My sister likes singing,too.(合并成一个句子) 

My sister and I singing.

76. Study hard,and you'll pass the English exam. (改为同义句)

 study hard, pass the English exam.

75. Huang Lei isn't as good at tennis as Larry.(改为同义句) 

Larry is tennis Huang Lei.

74. It will be fine tomorrow. They will go to the park.(改为含有条件状语从句的复合句) 

If it fine tomorrow,they to the park.

73. He has shorter legs than Sam.(改为同义句) 

His legs are than.

72. There's going to be an English speech contest next week.(改为同义句) 

  an English speech contest next week.

71 . The problem is so hard that he can't solve it.(改为同义句) 

The problem is hard for him solve.

70. Wang Tao is the tallest boy in our class.(改为同义句) 

Wang Tao is   student in our class.

69. I went to school on foot yesterday.(改为一般疑问句)

 you to school on foot yesterday?