把黄油放在冰箱里会变硬。

The butter will ___harden if you put it in the fridge.

生活的挫折让她变得坚强起来。

Life's setbacks have ___hardened her emotionally.

学校决定加强对学生手机使用的管理规定。

The school decided to ___harden its rules on students' phone usage.

随着谈判的进行，他对这个提议的态度变得越来越强硬。

His stance on the proposal ___hardened as the negotiations progressed.