Past Perfect and Present Perfect Tenses

The present perfect tense says that an action was completed at a time before the present, and the results or consequences of the action are relevant now. The present perfect is formed using the present tense of the verb "to have" and the past participle of the main verb.

The past perfect tense says that an action was completed at a time before another action happened in the past. The past perfect is formed using the past tense of the verb "to have" and the past participle of the main verb.

There was no sign of a taxi although I _______ it half an hour before.