他们是不是一直在我背后说我的坏话？

Have they been saying things about me ___behind ___my ___back?

得知这个决定是在她背后做出的，她感到被背叛了。

She felt betrayed knowing that the decision was made ___behind ___her ___back.

他们瞒着他安排了派对，想给他一个惊喜。

They arranged the party ___behind ___his ___back to surprise him.