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It is said that if we mention the time of happening of a past act, simple past tense should be used; and if we mention something happened some time in the past which still has certain effect at present, then present perfect tense should be used. In this regard, I find it difficult to write a correct sentence if I want (1) to identify the time of happening on the one hand, and (2) to emphaize that the act has been carried out on the other.

None of the following seems satisfactory:

Option A) I sent the confirmation letter by post on March 3: grammatically correct but it misses out my emphasis on the completion of the act.

Option B) I have sent the confirmation letter by post: grammatically correct but it misses out the time of happening.

Option C) I have sent the confirmation letter by post on March 3: seems grammatically wrong but it truly reflects what I want to say (the dual intentions mentioned above).

In this case, how should I say?
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You have a misconception of which you must disabuse yourself before you can feel comfortable with Option A!!!

The simple past is perfectly good at indicating the completion of an act! It's even very good at emphasizing completion. There is no requirement to use present perfect because an act is complete.

If you sent the letter, it is sent. You are absolutely finished sending it.
If you have sent the letter, it is also sent. You are absolutely finished sending it.
There's no difference between the two regarding completion.

The inclusion of a specific time trumps all other rules regarding the choice you are making. You cannot use present perfect with a specific time in the past, so you're right about Option C. It isn't really an option at all!

CJ
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Ms/Mr Wein is correct in her/his assessment of the present perfect. Jim is also right that the simple past can be used. But in a letter, where one wants some degree of formality or if one wants to stress the importance of such an action, the present perfect is used more often than the simple past.

In order to achieve the result you seek, Wein, it is normally necessary to do this in two sentences. I say normally because there are a few very specific instances where present perfect is used with a specific past time referent.

"I have sent the confirmation letter by post. It went out on March third."

ENLs normally 'set the stage' with the zinger, present perfect, and then they continue on with the simple past.

To achieve the result you've described, we find that this pairing of present perfect with a specific past time referent is becoming more common in speech. But with writing, the tendency is still, overwhelmingly so, to keep the present perfect and specific past time referents separated.
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Comments  
Thank you.

Maybe, one of the solutions to take care of the "dual intentions" is to add an "already" in between, i.e.,

"I already sent the confirmation letter by post on March 3."
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Personally, I wouldn't go that way. "already" is more typical with the present perfect. The two-sentence approach was a better idea.

CJ
I wouldn't go that way either, Wein but for slightly different reasons. "I already sent" sounds too informal but more importantly for the sender, it sounds like the sending is being downplayed when what you seek is to make it sound important.

I also understand where Jim is coming from when he suggests that 'already' is more often used with the present perfect. The 'rule' suggests this should be [also that 'just' should be used with the present perfect] but I'm not completely certain that this is the case for NaE.

These Google searches point up what I've suggested.

30,900 English pages for "I already sent".

24,200 English pages for "I've already sent".

28,600 English pages for "I have already sent".