The Past Perfect Continuous (PPC) is used to talk about a continuous event in the past which happened before another event to which it was closely related.

In this diagram, the wiggly line is the PPC and the cross is a closely related second event which “stops” the continuous event.

So to describe an ongoing action that started in the past and carried on until a certain point in time in the past we can use the PPC:

Drought-stricken farmers had been praying for rain for weeks and then it began to pour down.

She told me she had been dreaming about me lately.

The new underground station finally opened! Workers had been renovating it for weeks.

Time Reference

Usually you will find a time reference with this verb form. This can be very specific like:

since Monday

for 4 hours

before 3 pm

or more generic like:

since the beginning of time

for quite some time

before lunch

Reason

Another use of the Past Perfect Continuous is to show the reason for the outcome of an action that took place in the past.

They failed their test because they had not been studying.

We finally got married after we had been courting for several years.

Formation

The Past Perfect Continuous is formed by using the past perfect of the verb to be + the present participle‏‎ of the main verb.

had been + {present participle}

Or more simply put, the Past Perfect Continuous is formed in the same way as the present perfect continuous except using the past form of the verb to have.

had + been + verb-ing

They had been sleeping for hours when the barking dog woke her up.

Mary had been searching for some time before she found her purse.

To make the negative form just add not after had. You can use the contracted form too.

Tom had not been working very long for Heinz when the plant closed.

We hadn’t been reading much poetry until the new supply teacher came.

To ask a question simply apply the rules of question‏‎ formation with auxiliaries.

Had you been waiting long before your mum came to pick you up?

How much had they been drinking when they were arrested?