The past perfect tense

The past perfect tense denotes action that was completed before some definite time in the past. The past perfect tense is formed by combining the auxiliary verb had with the past participle of the principal verb:

had worked, had known, had been, etc.

In the following sentences, verbs in the past tense are underlined. Verbs in the pasr perfect tense are in italic.

By the time the police arrived, the thieves had disappeared.
The agent had sold all tickets before I applied for mine.