What is the present perfect?
The present perfect tense has many uses.  One use is to describe an experience (or lack of an experience) from an unspecified point in time. For example:

    I have visited Russia.
    I haven't visited Portugal.

The first sentence more or less means that you have the experience of visiting Russia. The sentence may mean you went there once, maybe twice, or even many times.  We don't know the exact time, only that it has happened. It is your experience. The second sentence means you don't have the experience of going to Portugal.

What is the sentence structure?
subject  |  have  |  past participle  |  object/complement

    I  |  have  |  swum  |  the English Channel.
    I  |  have  |  met  |  the Australian Prime Minister.
    I  |  haven't  |  finished  |  medical school.

Note that you can add the number of times you've had the experience, too. For example: 

    I have swum the English Channel twice.
    I have met the Australian Prime Minister many times.
    I have visited Brazil only once.

However, you can't give a specific date. Instead, we would use the simple past to express a specific point in time. Take a look at the following:

    X     I have met the Australian Prime Minister on August 25th, 2007.
    O     I met the Australian Prime Minister on August 25th, 2007.

In addition, we often use the adverbs ever and never when talking about experiences. Most of the time, ever appears in the question or the negative answer. On the other hand, never usually appears in the answer only. Never sometimes appears in written English, and less often in spoken English.

    A:  Have you ever studied German?
    B:  No, I haven't ever studied German.  -or-  No, I have never studied German.

Note that never is a negative adverb, so the auxiliary verb have is positive.

How is the present perfect used?
When talking about past experiences, the present perfect is used to ask a question and to provide the answer.  It's important to understand that when providing additional information, the simple past tense is used thereafter.  For example:

    A:  Have you ever visited Russia.
    B:  Yes, I have (visited Russia). I went there last summer for two weeks. I stayed for...

    -or-

    A:  Where have you traveled overseas?
    B:  I've been to Russia, Poland, Germany, and Brazil.
    A:  Which country did you like the most?

Is there additional information on the present perfect tense?
No, that's it!

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