PAST PERFECT TENSE

The past perfect expresses an activity that was completed before another activity or time in the past.

•All of my cousins had already left by the time I got home.

•Until yesterday, I had never heard about the accident.

PRESENT PERFECT TENSE VS PAST PERFECT TENSE

Present Perfect Tense

a. Who are those boys?

    I have never seen them before.   

b. Kevin is not hungry.

     He has just had lunch.

Past Perfect Tense

a. I didn't know who the boys were.

    I had never seen them before. (=before that time)

b. Kevin wasn't hungry.

    He had just had lunch.

PAST SIMPLE TENSE VS PAST PERFECT TENSE

Past Simple

A: Were your parents at home when you arrived?

B: Yes, but they left soon afterwards.

Past Perfect Tense

A: Were your parents at home when you arrived?

B: No, they had already left.