PAST PERFECT AND SIMPLE PAST

The past perfect tense is used to talk about two actions both completed in the past. The first action uses the auxiliary verb "had" with the past participle (3rd form of the verb). Example: had eaten
The second action uses the simple past.
Example: He had written an email before the computer stopped working.
1st action=had written 2nd action stopped

EXERCISE 1
EXERCISE 2
EXERCISE 3