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 B. When we use the peresent perfect, there is a connection with now. The action in the past has a result now.
    (����� ��������� ������ ������하������? �������� ������ ������ ��������� ���������.. ��� ��������� ��������� ������ �������� ���향��� ���������. ������ ���������..;;)
    ex) He told me his name, but I've forgotten it.(I can't remember it now.)

    We often use the present perfect to give new information or to announce a recent happening.(��������� ���������, ��������� ��������� ������할������ ���������������.)
    ex1) The road is closed. There's been an accident.
    ex2) (from the news) The police have arrested two men in connection with the robbery.

    We also use the simple past some situations.( ... ������������ ������..ㅠ.ㅠ ��������� 확���하��� 한��������� ���������..;;)

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 C. We ofte use the present perfect with just, already, and yet. You can also use the simple past.
    (������ just,already,yet��� ������ �������������� ���������, ������������������ ������ ���������. ���������--;)
    Just = a short time ago.
    ex) I've just had lunch. or I just had lunch.
 
    We use already to say that something happened sooner than expected.(������했������������ ��� ������ ��������� ������������ ������)
    ex) I've already mailed it. or I already mailed it.

    Yet = until now. It shows that the speaker is expecting something to happen. Use yet only in questions and negative sentences.
    ex1) Has it stopped raining yet? (simple past��� ������!)
    ex2) I wrote the letter, but I haven't mailed it yet. (������ simple past ������)

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 D. Don't use the present perfect when you talk about a finished time.(������ ��������� ��������� ����� ������ ���������������. ������한 ������)
    ex) It snowed last night.(not has snowed)

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 B. We use the present perfect with today/ this morning/ this evening, etc. when these periods are not finished at the time of speaking.(������������;;)
    ex) Have you had a vacation this year?

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 C. We say, "It's the first time something has happened."(present perfect) (������ ���하������ ������..;;)
    ex) Sarah has lost her passport again. It's the second time this has happened.(not happens)