therein

副词 therein:

1. (formal) in or into that thing or place
   同义词：in this, in that

1. Therein lies the crux of the matter.
   问题的关键就在那里。
2. He is not a snob and therein lay his downfall.
   他不是个势利小人，而他失败的原因正在于此。