Verb tenses : Past pefect     
   
Form of the Past Perfect
   The past perfect tense is often used in English when we are relating two events which happened in the past. It helps to show   which event heppened first.
  It is formed with the auxiliary verb have in the past (HAD), followed by the participle of the main verb.
 

Example sentences:    I had finished my homework.

                                      He had been in USA since 1999.

--The past perfect is used to show you which of two events happened first. Imagine that two things happened in the past:

      Examples:  I had studied english before I travel to USA.

                         She had left the house when we arrived.