题目内容
18．每个人都会犯错误，但是吸取教训是非常重要的．Everyone can make mistakes，but it is importantto learn from them（learn）．
分析 to learn from them．
解答 答案：to learn from them．结合句意是说做某事是重要的，结合it is important用it's+adj+to do sth，it 是形式主语，动词不定式是真正主语，表示吸取教训用learn from the mistakes从错误中学习，用them替代mistakes，故答案是to learn from them．
点评 形式主语，顾名思义，即形式上是主语，但实际上不是真正的主语，真正的主语是后面的动词不定式、动名词或主语从句等．如果让后面的动词不定式、动名词或主语从句用作主语，那么在句式上容易造成"头重脚轻"，句式不平衡，故往往用it作形式主语，而把真正的主语放在后面．it在这里没有任何意义．
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