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语法拓展
疑问代词或副词后的不定式
疑问代词 who, what, which 等和疑问副词 when, where, how 等后面跟不定式，构成不定式短语。这种结构在句中可作主语、宾语、表语、宾语补足语等句子成分。此结构可以改写成由该疑问词引导的从句，从句所作的句子成分不变。如：Perhaps the most difficult challenge is how to survive without friends. 可以改写成 Perhaps the most difficult challenge is how we survive without friends（也许最艰难的挑战，就是生活中孤独无友。）
