present perfect tense幾時用???急

Present Perfect Tense

簡易記法是：過去 + 現在。

用於以下 3 種情況：

1 剛發生的事情 (仍屬現在中的過去)：
He has just left. Should I call him back?

2 已發生了但仍影響到現在的事情：
I have broken my leg. Can you help me to get up?
腿傷發生於過去，但現在仍不良於行。

若事情對現在已無影響，則應用 Simple Past Tense.
I broke my leg last month, but I feel perfectly OK now.
腿傷發生於過去，對現在已無影響。

3 從過去到現在共發生了多少次：
I have seen him here many times.
He has loved only one woman throughout his life.
She has never danced on the stage.


參考資料 http://hk.knowledge.yahoo.com/question/question?qid=7009101700542

present perfect tense use in:

1.already(has do that), yet(hasn't do that)

Example:
a. I have already done my homework.

b. Flaska hasn't bought the conical flask yet.

c. Unununium has been discovered already.

--------------------------------------------------------------------------------------------
2. Have you ever / How many times

Example:
a. Have you ever used litmus paper?
No, I haven't.

b. How many times have you mixed solutions?
I have mixed solutions once.
--------------------------------------------------------------------------------------------
3. Hidden "already" or "yet" meaning

Example:
a. Helium has been discovered.

This has three kind of times may use:
1) already completed still has the connection movement or the condition with present;
2) already starts in the past and continues to the present movement or the condition;
3) has experienced movement or condition.