The Past Perfect Tense

Affirmative
I had worked.
She had worked.
They/you/we had worked.

Negative
I had not worked.
She had not worked.
They/you/we had not worked.

Interrogative
Had I worked?
Had she worked?
Had they/you/we worked?

The Past perfect tense denotes an action completed at some point in the past before some other past action commenced. When two actions in the past have to be referred to, the past perfect is used for the earlier action and the simple past for the later one.

John had taken two wickets before rain interrupted the play.
I had seen him twice before he left for Paris.
The train had left before we reached the station.
He had left before I reached home.
The sun had set before the traveler reached his destination.

The Past Perfect Continuous Tense

Affirmative
I had been writing.
She had been writing.
They/you/we had been writing.

Negative
I had not been writing.
She had not been writing.
They/you/we had not been writing.

Interrogative
Had I been writing?
Had she been writing?
Had they/you/we been writing?

The past perfect continuous tense is used to talk about an action which started some time in the past and continued uninterruptedly till a later point of time in the past when some other development took place.

Consider the sentence given below:

I had been reading till midnight when the lights suddenly went out.

In the sentence given above, I started reading some time in the past, continued till midnight and was still continuing when the lights went out.

The war had been going on for two years when John enlisted.

The war started some time in the past, continued for two years and was still continuing when John enlisted.

I had been giving him financial help for three years when he got a good job.

I started giving him financial help some time in the past, continued for three years and was still continuing when he got a good job.

Note that it is the duration and continuity of the action up to a point of time in the past that is stressed by the past perfect continuous tense.