關於英文的tense

we use PRESENT TENSE when things are obvious.
e.g The sun rises from the east.(this never changes)

we also use PRESENT TENSE when it is a routine.
e.g I brush my teeth everyday.(you do this everyday)

about pp=PRESENT PERFECT TENSE,
we use it when you finished an action in the past, and this situation is continuing.
e.g I have done my homework.(you finished your homework, and now: your homework is still finished[your homework will not suddenly become unfinished, right?])
e.g I have lost my key.(you still cannot find your key, your key is still gone at this moment, it will not suddenly appear)

for PAST TENSE,
you finished the action in the past, however you do not know whether the situation is continuing or not.
e.g I lost my key.(you lost it in the past, but maybe you have already found it now.)

we use PAST CONTINUOUS TENSE when a specific time of duration is told in the past.
e.g I was cooking yesterday night from six to seven. (XXX you do not say: I cooked yesterday night from six to seven.XXX)

I am a top band 1 middle-top student and English is my best subject(I may say)
Hence, the above information should be right. You can trust me, hopefully these things will be useful for you.

2013-06-04 17:28:40 補充：
if using your information,
PAST TENSE: I lived in HK. (meaning you lived in hk in the past, but the readers do not know whether you are still living in HK or not.)

2013-06-04 17:35:19 補充：
PRESENT TENSE
1.describing sth is true in general-water is liquid.(Xwater was liquidX)
2.your ability-he drives a bus.
3.your how often you do sth-I have dinner every night.

2013-06-04 17:37:07 補充：
we should not use present tense,
we should use PRESENT CONTINUOUS TENSE: I am living in HK(because you are really living there at present/now, but living in HK is neither a routine, nor your ability---> so we do not use present tense)

I have uploaded a video about Tense on Youtube. You can find the answers to your questions there.

http://www.youtube.com/watch?v=y6ED1dgXkgg

首先，I have lived in Hong Kong Kong 和I live in Hong Kong 是完全不同的。
I have lived in Hong Kong Kong 就是你以前住香港，I live in Hong Kong 是你現在住在香港。兩者的分別從結構到用法都不同。

Simple present tense 是在你陳述事實(Ice melts under the sun)，或者是描述一個正在進行的行為(we eat biscuits)， 或一個習慣性行為(He plays badminton every friday)用的。口語上也能作future tense 用(The plane arrives the day after tomorrow)。

Present perfect tense 是在描述一個從過去到現在還在發生的事件(I have been playing the piano since 3 o'clock this afternoon)，或者是一個剛完成的事件使用的(I have just finished playing piano)。

Simple past tense 和Past continuous tense 的分別在於一個是用來描述在過去的一段長時間所發生的事，和一個已經在過去完結的事件。

I was showering when the fire alarm rang.

這個例子很明確的解釋了兩者的分別。 i was showering是一個持續了一段時間的行為，fire alarm rang- 是在某一刻裡發生的，所以要用simple past tense.