他感觉自己困在毫无挑战的工作中，一成不变。

He felt stuck ___in ___a ___rut at his unchallenging job.

许多人担心，没有新的机会，自己的职业生涯会陷入停滞不前的状态。

Many people fear their career will get stuck ___in ___a ___rut without new opportunities.

经过多年不变的生活作息，我的社交生活彻底陷入了一成不变的窠臼。

After years of the same routine, my social life was firmly ___in ___a ___rut.