about ENGLISH既tense

past perfect tense
indicates an action that preceded another action in the past
had+come /eaten......

present perfect tense
1.actions completed in the past that have some consequence or effect on the present situation
2.to continuous actions begun in the past and extending into the present
have/has+come/eaten.....

past tense 與 present perfect tense 分別
I did my homework yesterady ago (講明時間)
I have done my homework (無講明時間 - 通常有already, just, since, for等字眼)

past perfect tense (講明時間,通常與past tense 一同用,用作先後之分,先past perfect tense,後past tense)
After I had done my homework, I went out.