ADV
此中;在这里面
You can use herein to refer back to the situation or fact you have just mentioned, when saying it is something such as a problem or reason for something.
例句
The point is that people grew unaccustomed to thinking and acting in a responsible and independent way. Herein lies another big problem.
关键是人们已不再习惯以一种负责任的、独立的方式去思考和行动。这其中就存在另一个大问题。