1. The plan's success was balanced on a knife-edge.
计划能否成功还很难说。
1. knife-edge的意思
1. 形势难料；结果未卜
To be on a knife-edge means to be in a situation in which nobody knows what is going to happen next, or in which one thing is just as likely to happen as another.
e.g. His future remains on a knife edge...
他前途未卜。
e.g. The game is poised on a knife-edge. One mistake or one piece of good luck could decide it.
比赛形势扑朔迷离。一个失误或是一点运气就有可能决定胜负。
2. 形势难料的
You can use knife-edge to refer to something that is very exciting or tense because you do not know what is going to happen next.
e.g. Tonight's knife-edge vote could be uncomfortably close.
今晚投票的结果很难预料，双方实力可能会很接近，这令人紧张不安。
noun
1. a narrow boundary
e.g. he lived on a knife-edge between genius and insanity