我譯英？？？

Nobody knows the hidden reason of what he has done!
After a series of training, I am sure you have already learned the relevant management skills!

Have not known he does the reason behind this!
沒有知他做這件事的背後原因！

Through a succession of training, it is believed that you have already understood relevant management skills!
經過一連串的訓練，相信你已懂得相關管理技巧！

Don't know the reason behind why he have done this!
We believe you have learned the management skills after the training.

Don't know the reason behind why he did this.
After a period of training time, trust that you already know the technique of management.

It is unknown to people why he has done this.

After a series of training, you are believed that you have learned the management skills.

Translation according to my knowledge:
沒有人知他做這件事的背後原因！
Nobody knows the reasons behind for him to do so.
經過一連串的訓練，相信你已懂得相關管理技巧！
After the series of training, I believe that you have understood the relevant management skills.