Trouble 放在片語裡，成為慣用的既定用法，表達其語意「概念」，就不必強調「個體性」，像是 have trouble、in trouble 或是 get into trouble 等。然而，如果要明確指出一項個別的「煩惱」，也就是強調其「個體性」的用法，就需要有單複數的標記：
Don’t get yourself into trouble.
Every time he is with his older brother, he always gets into trouble.
She has trouble focusing on her work when she drinks too much coffee.
= She has difficulty focusing on her work. → 片語用法，不強調個體化
Please do not take the trouble to travel to Kaohsiung.
The most comforting thing about having close friends is being able to talk about our troubles.
→ 指各人的種種煩惱，強調個體化的 troubles