past perfect & present perfect

所有perfect tenses既句子都係敘述緊兩件事情，而發生早一點的事情便要用perfect tenses，發生後一點的就用simple tenses.

Present Perfect Tenses(一件事係o岩o岩過去，一件事係發生緊):
發生早一點的事情: have/has + p.p.
發生後一點的事情: verb in simple present tense
e.g. I put down the pen when I have finished English homework.

Past Prefect Tenses(兩件事都係過去了的):
發生早一點的事情: had + p.p.
發生後一點的事情: verb in simple past tense
e.g. I went to bed when I had finished my revision last night.

Present Perfect:
(Active=have/has+PP)
eg1.I have studied English in several different countries.
eg2.I have studied a Clarian since F.1
(Passive=have/has been +PP)
eg1.the windows have been cleaned by me already.

Past Perfect:
(Active=had+PP)
eg1.I had studied a little English before I moved to the U.S
eg2.I had studied a student in St.Clare's Primary before i became a F.1 student.
(Passive=had been+PP)
eg1.The windows had been cleaned by me before you came.

Present Perfect:係岩岩
Past Perdect:係過去左既再之前
留意下D example..你就明白有咩different
全部都係我親手打,,絕無抄人成份
hope i can help you