Present perfect and Past simple

Firstly, I want to say that your box is really interresting and constructive. I often come here to read your posts and to learn English. It''''s really very helpful for english learners.

Secondly, I have a question that may sound stupid for most of you here but I will post it anyway and hope that some one can help me.

I really don''''t know why I always have difficulties to to distinguish the Past Simple and the Present Perfect. When I read grammar rules about this, I think I understand. But in daily conversation or email, each time I have to talk about smt in the past, I don''''t know which one to use in each different situation.

If any one of you can give me some "tricks" so that I can easily distinguish these two things, I would be thankful.

For your information, english is my second foreign language and in my first foreigne language, there''''s not Present perfect.

Thanks

Có những dòng sông không trở lại bao giờ ....

Được dongsongkhongtrolai sửa chữa / chuyển vào 10:43 ngày 31/03/2004

I don''t understand this sentence. "... and in my first foreigne language, there''''''''s not Present perfect" --> Do you assume that we (not including you) are not Vietnamese?
Anyway, this is trick number one: if the sentence has time markers then use Simple Past, otherwise use Present Perfect. Time markers include: ago, yesterday, last night, last year, etc. Also note if there are "Present Perfect indicators." Some of them are: since ( + time), for (+time), the past few years, the last two years, yet (meaning "chưa"), etc.
All boils down to this: Simple Past Tense is used with a specific time in the past whereas Present Perfect Tense is used with an unspecific time in the past. The action described in the Present Perfect has some effect in the present.
Ex:
I came to America one year ago.
I have not come to America for 4 years. (Now I am not in America)
It has just rained heavily. (unspecific time, also you see the ground wet)
Simple Past (action started and ended)
----X--------- Present
Present Perfect (action started in the past, but did not end in the past but last up to the present)
-----XXXXXX-Present
Hope this helps. Have a nice day!
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Thank you nktvnvn , your explanations are clear.
But I still have a question : I have the following sentences :
1. I''m sorry, I haven''t noticed that you start your paragraphs with ...
2. I''m sorry I didn''t notice you were not happy.
3. I?Tm sorry I didn?Tt notice this post sooner
4. I''m sorry, I didn''t notice the phone ringing
5. I''m sorry -- I haven''t noticed that you''re a girl
Can you pls explain me why they use the present perfect in the sentences 1 and 5 and the past simple in the others ?
Có những dòng sông không trở lại bao giờ ....

The context is not very clear. Give me a context for each sentence and I''ll give you an explanation.
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Sorry, I can not give you the context for each sentence but I know they are all correct.
Could you please just take two if these sentences : one with past simple and one with present perfect , and explain me (by adding exemple of context ) the difference ?
Thanks in advance
Có những dòng sông không trở lại bao giờ ....

OK, let me try to explain one sentence.
I''''''''m sorry I didn''''''''t notice you were not happy.
A possible context is:
- You were unhappy yesterday (in the past) and I did not notice that -> this means that the action "notice" started and ended yesterday. I know for sure that now you are not unhappy anymore. (It''''s too late for me to notice your unhappiness because you''''re not unhappy anymore. )
- If you say I''''m sorry I haven''''t noticed you are not happy. then you mean that I am still unhappy. (and it''''s not late for you to notice that, )
Notice the tense after the Present Perfect and the Simple Past Tenses. I hope you''''ll see the difference:
5. I''''''''m sorry -- I haven''''''''t noticed that you''''re a girl
As discussed earlier, the Present Perfect is used with an unspecific time. Once the time becomes specific, you must use the Simple Past. This example may make this point clear:
A: Have you ever been to a wedding? [unspecific time]
B: Yes, I have.
A: Whose wedding was it? [the time is now specific. It is understood but not stated]
B: My brother''''s.
A: Have you ever been to a wedding?
-XXXXXXXXXXX-Present (A uses PP to mean ''''from the past up to now'''')
A: Whose wedding was it?
-----------x-------Present (A uses SP to refer to the time at the wedding of B''''s brother. In fact, A cannot use PP here.)
Are you still confused now? You''''re in good company. The two tenses, in some cases, are not so easy to distinguish. I have trouble with them myself sometimes!
Được nktvnvn sửa chữa / chuyển vào 03:39 ngày 03/04/2004

First thing first, I think she meant that she has learnt to speak two languages apart from Vietnamese, and English is the second, her first (French? Italian? Chinese? ) doesn''t have "present perfect" tense.
But anyway..on with the point of the question.
I''m not sure if this will help you but this is how I see it.
"I haven''t noticed that you were unhappy..." means that ''till NOW, ''till the MOMENT HE SAYS THE SENTENCE he indeed has not noticed that she was unhappy.
"I didn''t notice that you were unhappy" means that in the PAST, not necessarily ''till the moment he says the sentence, probably yesterday, last year, two hours ago..etc..that he did not know she was unhappy.
So ultimately, the difference between two tenses is that ''has not'', ''have not'' etc..present perfect means: ''till the present, ''till the moment the sentence is spoken, or close anyway;
There are cases like "I hadn''t noticed..etc.." implies the ''present'' year, week (a longer period of time).
Whilst "did not" purely means PAST PAST PAST PAST PAST , sometimes long ago, yesterday, an hour ago, a week ago, last year..etc..
^^;; I hope this helps.
"Gomen nasai....demo..A****erui Sayuri-san.."

What a head ache!
I have only one rule
- When I think about the past and present together, I normally use the present perfect.
FINISH ACTION---------PRESENT PERFECT--------------------PRESENT
letter (yesterday)----->I''ve written to J<--------------J knows now
cake (this morning)------->I''ve made. do u want some?<----I''m offering u some now
-When I think only about the past, I most often use simple past
My grandmom wrote me a lot of letters.
(She''s dead now, I''m not thinking about the present.)
I made a cake for the children, but they didn''t like it.
(I''m not talking about the present.)
hopefully this helps u a little bit out
oops there''s one note that you should remember. We don''t normally use the present perfect with words for a finished time
ex: I''ve seen her or I saw her (do not say: I''ve seen her yesterday)
Được invincibleguy sửa chữa / chuyển vào 00:11 ngày 04/04/2004