2020-2021学年高二同步新教材人教版（2019）选择性必修第三册学案UNIT 2 突破语法大冲关

不定式作主语 【观察例句】 1．To say is one thing and to do is another. 2．It's rude to turn your back to your teacher and refuse to answer. 3．It is impossible to learn a foreign language without making painstaking effort. 4．It seemed a pity to have wasted so much time. 【归纳用法】 1．把不定式置于句首。如： To get there by bike will take us half an hour. 骑自行车到那儿将花费我们半个小时。 To make a plan first is a good idea. 首先制定好计划是个好主意。 2．用it作形式主语，把真正的主语不定式置于句后，常用于下列句式中。如： (1)It＋be＋名词＋to do It's our duty to take good care of the old. 照顾好老人是我们的责任。 (2)It takes sb.＋some time＋to do How long did it take you to finish the work? 你花了多少时间来完成这项工作？ (3)It＋be＋形容词＋for sb.＋to do It is difficult for us to finish writing the composition in a quarter of an hour. 我们难以在15分钟内完成这篇作文。 (4)It seems(appears)＋形容词＋to do It seemed impossible to save money. 省钱似乎不可能。 (5)It ＋ 不定式结构可位于believe/consider/discover/expect/ find/think和wonder之后 He thought it would be safer to go by train. 他认为乘火车比较安全。 (6)不定式的完成式也可作句子的主语 To have made the same mistake twice was unforgiva