Past perfect continuous

Form: subject + had been + ing

{Positive}
I, We, They, You, He, She, It    →     had been waiting.    
 
{Negative}
I, We, They, You, He, She, It   →      had not been waiting.
 
{Questions}
 Had  →   I, We, They, You, He, She, It been waiting?
 
Key words:
Since, how long, all day, for.
 
Uses:
If one talks about a duration before an action or an event that occurred in the past.
Oliver had been studying at London university for one year when he decided to drop out.
Mohammad wanted to take a siesta since he had been working.

If one talks about the cause of an event or action which occurred in the past
I was fatigued and exhausted therefore I had been walking.