Definition :
Action in progress immediately before, up to, until another time or action in the past
Pattern :
+ S + Had + Been + VI(ing) + O
S + Had + Not + Been + VI(ing) + O
? Had + S + Been + VI(ing) + O
Examples :
+ They had been singing, when we studied dancing
- They had not been singing, when we studied dancing
? Had they been singing when we studied dancing?
+ I had been writing the lesson when Rudi met me
- I had been writing the lesson when Rudi met me
? Had I been writing the lesson when Rudi met me?