"Shoulder" has a "s" because her mother put her hands on both shoulders. And because people have two shoulders.

We had been playing when the bell rang. It’s the past perfect continuous(progressive) tense. It shows that an action that started in the past continued up until another time in the past. So the "playing" stopped when the bell rang. *********** We were playing when the bell rang. It’s the past continuous verb tense. It shows action at a particular moment in the past. The action started before that moment but has not finished at that moment. So the "playing" may have continued when the bell rang.