问题标题:
Andifyoubecomerich,youwillhaveadifficulttimeknowingwhoyourrealfriendsare.这句话为什么用knowing,
问题描述:

Andifyoubecomerich,youwillhaveadifficulttimeknowingwhoyourrealfriendsare.

这句话为什么用knowing,

崔明慧回答:
  havesth+doing表主动语态havesthdone表被动语态
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