Some confused english grammar problems...

I am not good at present perfect tense.The usages of this tense is: 1.Actions which started in the past and are still continuing 2.Actions which happened at some unknown time/unspecific time/unfinished time in the past,which means
it's important to say that something happened (or didn't happen).3.Actions which happened in the past, but have an effect in the present.

Here are some problems:I have written six letters this morning. It means I have finished writing six letters,right? but how to figure it out? why it doesn't mean like the usage 2, i wrote 5 letters before, and now i am still writing letter, and this letter is the sixth letter.( we know we won't write 6 letters at the same time, we write one letter once, then we write another letter),is that because "this morning" is an unspecific time/ an unfinished time(suppose it is 9:00 A.M now,it is still morning,not finished),it is not a duration time? I have been writing letters all morning. it means i wrote some letters before, and still writing LETTER now.(present perfect continuous tense has only this usage) i think "all morning" is a duration. but can i say I have been writing six letters all morning, and it means i wrote 5 letters before,and now i am writing the sixth letter? Can i say I have been writing letters this morning? Can i say I have written letters all morning? does it mean i wrote letters before and still writing it? Can i say I have written six letters all morning? it means i wrote letters before,and still writing letter now? Can i say I have written letters this morning? it means i wrote letters before?     And I am not sure if the negative present perfect sentence has the same usages as the present perfect statement's. Like,I have eaten chicken,it means i ate chicken. I haven't eaten chicken. what does it mean? it means i didn't eat chicken before (usage 2/3) Or I didn't eat chicken and not continuing,not eating now(usage 1) Or it depends on the context.

I hope you can answer those questions, maybe you feel confused,but please look at it very carefully,then you will know what i mean! Thanks.

Hello MIA6

Are you a native speaker? If so it's very interesting to me. I'm a Japanese, who have learned English rather for a long time but still find it tough to catch the exact sense and usage of English present perfect tense. I'm afraid you might feel ill to hear about the grammar of your native tongue from a foreign person who is learning it, but if you don't mind, please read with a laugh wondering in what a queer way an English learner from Orient understand your mother tongue.

As far as I have learned and as far as I have understood, the basic notion of the English expression "I have done something" is "I now have the result of an experience that I did it in the past".  

So "I have written six letters this morning" is "I have now the result of an experience that I wrote six letters this morning". It means you have already finished writing all the six letters. "I have been writing letters all morning" means "Now it is still morning but this morning I wrote some letters and still I am writing a letter".

"I have been writing six letters all morning" sounds a bit weird. "Write letters" can be used to express an activity of writing some letters in a generic sense but "write six letters" sounds to be any specified activity such that you should know what letters the six letters are. So, if I hear "I have been writing six letters all morning", I take it as "This morning you began writing six letters in parallel and you are still continuing the six paralleled activities of writing a letter".

"I haven't eaten chicken" may be commonly taken as "I have the result of an experience that I never ate chicken in the past". Suppose your mother asked you in the kitchen "did you snitch the chicken?" and suppose you answered like "No, I haven't eaten the chicken". This means "I didn't yet eat the chicken this time". The use of THE before "chicken" gives to "eat" a notion of a specified activity.


paco

[PS] When I finished this writing, I noticed CJ had already posted a much better answer. But I will dare to post it hoping it can be served as a material for you and other native speakers to know what understanding an ESL student has about English present perfect tense.

CalifJim has written an excellent summary here. May I just add a couple of observations of my own.

CalifJim wrote:

You may be driving yourself crazy trying to make every sentence with a present perfect tense fit into one those three categories!  I have written six letters this morning. This means that during the morning you finished the task of writing six letters.   You are not still writing the sixth letter.  You are not writing anymore.  You are now just sitting there, perhaps, looking at what you have accomplished during the morning and you see the six letters on the desk.  You are saying where you stand with respect to letter-writing at this very moment.  You are saying that at this moment you have six letters before you, all written.

I think it's important to make it clear here that we should only use the above sentence in the present perfect tense if it is still "this morning". In other words, there must remain the potential for me to write more letters this morning. If it is now evening, I must say "I wrote six letters this morning" (in the imperfect tense) because I cannot now write more. This is the 'present' element of the present perfect tense: it is "this morning" now, when I say it. The 'past' element (and it is a past tense despite what some confusingly claim) is that the writing was done in the past, and is now finished (present perfect) or continuing (present perfect progressive).

CalifJim wrote:

I have been writing letters all morning. This says what activity you were involved with all morning, that is, during the entire morning.  It is still morning and you may still be writing letters or you may have decided to stop now.  In either case, the sentence just says what activity you were involved with up until this moment of speaking.

This is the present perfect progressive tense, and it is subtly different. As Jim says, the letter-writing started in the past, but I am implying that it may still be continuing. Again I can only say this sentence this morning, not this evening, because I am referring to this morning as the time I write the sentence (not the letters).

CalifJim wrote:

It's easy to see how this topic can become very confusing! It would be better to ask shorter questions by dividing this up into several posts. CJ

With regard to I have been writing ..., you end your post with the question

so you mean i I have done six letters

The answer is "No".  When I explain have been writing in terms of were working on, I don't mean have done; I mean have been doing.

Maybe a point of confusion lies with the progressive tenses - the ones with the -ing forms.
If we can say that we were writing letters, and we continue writing them up to the moment of speaking, we can say we have been writing letters.
If we can say that we wrote letters, we can say that we have written letters.

were writing does not mean wrote.
have been writing does not mean have written.

were writing and have been writing both say that an activity was in progress, not that it finished.
wrote and have written both say that the writing finished.

CJ