詢問現在完成式既野

Basically, your sentence has found nothing wrong. However, it would be better if you use "JUST" to modify your sentence.

I have JUST thrown away the letter from Mary.

(1)The Present Perfect Tense indicates an action took place in the past,is associated with the present idea of now, eg:-I have thrown away Mary's letter. (Up to Now)
(2)The present Perfect Tense is used for an action just concluded when the resulting state is still present, eg:-
I have thrown away Mary's letter ; I am unable to read it anymore.
(3)With activities completed in the immediate past (00:01 sec.)just is often used, eg:-The letter has just thrown.
Note though just is an adverbial of the present, just now (= a little time ago) is an adverbial of the past and the verb used with it will be a Past Tense, eg:-Mary's letter threw away just now.(by me)
(4)For duration of an action or of absence of an action begun in the past and continuing to the present and possibly to the future,eg:-I have thrown away Mary's letter for (00:01 sec).
Compare this with;-
I threw away Mary's letter (and no longer need it).

「我知道現在完成式唔會講明個個動幾時做既。」
你條粉腸，知你又問。

2014-03-18 00:12:01 補充：
「我係十點零分零一秒向我妹妹講」
「我係早上十點丟左Mary封信」我肯定丟左封信後唔止過左一秒。

2014-03-21 02:03:06 補充：
朋友，同類問題你問了幾十次，我合理地懷疑你對tenses有強逼性心理障礙，建議你找精神科醫生檢查一下。