TENSES

其實英文文法既用法係有規定架~
所以好難答你點解～
不過我都可以解釋下既．．．

E.g.1: After he had brushed his teeth, he had breakfast.
逢係有"...before..." 或者 "...after..." 既句子,
就一定係用"Past Perfect+Past Simple"架啦
呢2個字係用黎表達2樣事情既先後發生次序
並且要發生係以前
同埋呢2個動作已經係完成左既
"Past Perfect+Past Simple" 即係：
首先做既動作用Past Perfect
~ After he had brushed his teeth,（佢首先刷牙）
後來做既動作用Past Simple
~ he had breakfast.（然後佢食早餐）

E.g. 2: She has been a DJ since she left school.
你要留意下呢句野入面既"since"字牙!
多數有"...since..(a time period)..."/ "...for...(a time period)..."/ ".....yet."
就會用到 "Present Perfect Tense"
呢個用法,
係一個由以前已經開始做既動作，
直至宜家都仲做緊既:
"自從佢畢左業離開學校之後，就一直做緊DJ."
所以要用Present Perfect Tense．

2008-01-08 13:18:23 補充：
如果你有任何有關Tenses既問題都可以"腥依貓"問下我架無任歡迎喔～**I'm LiTtLe TaMaMa**

"he had brushed his teeth" he has already finish brushing his teeth. "he had breakfast" he has already finish his breakfast so we use past perfect.

" She has been a DJ " because she is a DJ now too, not only in the past, so we use present perfect.
That means we use past perfect when the action is already happen and finished.
We use present perfect when the action has only happen, but it has not finish till now.

1. It needs to use past perfect tense because he is done with those actions. It was in the past.

2. It needs to use present perfect because she is still a DJ. She is still doing her job as DJ now.

For other example: She has been practicing piano since noon.
-----> It means she is still practicing piano since noon time until now at this moment.

or She has been teaching English for 10 years.
----> It means she is still teaching English now.