6. 带疑问词的不定式短语
　　不定式前可以加某些疑问代词，如who、what、which,或疑问副词，如when、where、how、why等，构成一种特殊的不定式短语，可在句子中作主语、宾语、表语或状语等。

　　How to improve English is often discussed among the students.
　　We haven’t decided when to visit the place.
　　The most difficult thing in learning English is how to speak the language well.
　　You haven’t answered my question where to get these books.