stiffen|stiffened|stiffening

学校决定加强纪律规定以改善学生行为。

The school decided to ___stiffen the discipline rules to improve student behavior.

冷空气让面团变得有点硬。

The cold air is ___stiffening the dough a bit.

她用硬麻布加固了衣领，让它更挺括。

She ___stiffened the collar to make it more rigid.