81. 如果不是因为他的帮助，我们不可能克服重重困难
82. 是我们完成工作的时候了。
83. 不幸的是他们没能考虑到交通拥挤，到机场时太晚了，没能赶上那次航班。
84. 你一旦定出了一个学习计划，就应该把它付诸实践。
85. 他希望在周末能赶上他落掉的课程。
86. You can make your dictionary use much more efficient if you only look up the words which are necessary to understand the text
87. Even if you can t work out the meaning exactly, you may be able to get a vague idea, enough to enable you to continue reading.
88. He said that setting aside time for improving his physical fitness would be a total misuse of his working hours.
89. By watching and learning from a model, you will probably begin to identify and adopt good working habits.
90. He collected much money from his friends and posted it to the people in the earthquake area.
参考答案：
81. Had it not been for his help, we couldn t have overcome all the difficulties.
82. It is time that we finished our work.
83.Unfortunately they hadn t taken the heavy traffic into account and it was too late to catch their flight when they got to the airport
84. once you have worked out a study plan, you should put it into practice.
85. He hopes to catch up on the lessons he missed during the weekend.
86. 如果你仅仅是查阅一些对理解文章有必要的词，你可以更有效的使用你的词典。
87. 即便你想不出它具体的意思，但是你能够了解一个含糊的中心，足以能够让你继续阅读。
88. 他说留出时间为了改善自己的身体健康完全是滥用工作时间。
89. 通过的观察和学习，你们有可能将开始认同和采纳好的学习习惯。
90. 他向他的朋友们募集了许多钱并把这些钱寄给了地震灾区的人们。