Past Perfect or Past Perfect Continuous
The past perfect is used for single quick events
e.g. breaking a glass – I had broken a glass NOT I had been breaking a glass
For continuing / longer / repeating events it is possible that we may need to use the past perfect continuous:
e.g. writing a report (this takes a long time) / calling your friend when they do not answer the phone (sometimes you call your friend many times with no answer)
but we do not use past perfect continuous all the time with continuing or repeating events, in fact we do not use it so often, but sometimes we MUST use it, see the image below to see when we need to use past perfect continuous for continuing or repeating events, then why not test yourself with our exercises: Quiz, Gap-Fill, Match the Sentences, Worksheet.
Also, beware that there are some verbs which cannot be used in any of the continuous tenses. Some verbs will sound strange to you in the continuous form, and it is a good idea to learn what these verbs are: Verbs that Cannot be Used in the Continuous Tenses