重点讲解：
1. impose on 强制实行；强加；
The conditions imposed on volunteers were stringent.
对志愿者的限定条件非常严格iM7g=J^z=!zZ。
2. be intended to do sth. 想要；打算；计划；
The defences are intended to obstruct any advance by tanks and other vehicles.
这些防御工事用于阻止坦克等车辆前进S=;iN*qK*^BAef。
3. call off 取消；
He has called off the trip.
他已取消了这次行程=_6dFbtBVf]Q|qwu,rs。
4. threaten to do sth. (坏事)可能发生，将要发生；
The fighting is threatening to turn into full-scale war.
这场战斗有可能会演变为全面的战争=fHF4t|DHcrD。