a. Form: S + Had + Past Participle (PP)
The past perfect is used to express an action in the past, which happened before another action in the past. Ex: when I got home, John had cooked a meal.
Note: the use of past perfect and the past simple in the following sentences. Ex: when I got home, John had cooked a meal – (John cooked a meal before I got home). When I got home, John cooked a meal – (First I got home, and then John cooked).