* "After the argument, he sat in the corner with a sullen expression." (This sentence uses "sullen" to describe a facial expression.)
* "The storm clouds gathered, casting a sullen shadow over the landscape." (This sentence uses "sullen" to describe a mood or atmosphere.)
* "She gave a sullen nod, refusing to make eye contact." (This sentence uses "sullen" to describe a person's behavior.)
* "The silence in the room was broken only by the sullen ticking of the clock." (This sentence uses "sullen" to describe a sound.)
Remember that "sullen" typically suggests a mood of displeasure, gloominess, or resentment.