Let’s analyze the tenses of the sentence constructions in the
EnglishPage.com webpage that are bothering you:
1. “My best friend and I
have known each other for over fifteen years. We still get together once a week.”
The first sentence in the construction above is indeed in the present perfect. It is in the present perfect because the verb phrase “have known each other” conveys the idea that the action has subsisted or continued up to the moment of speaking, and the adverbial modifier “for over fifteen years” is actually not a specific time of occurrence of the action—this is the element that shouldn’t be in the present perfect form—but a period or duration. In contrast, when the time of occurrence of the action is specified (“yesterday,” “last week,” or “five years ago,” etc.), what’s needed is the simple past tense instead, as in this sentence: “My best friend and I first
met in a bookstore five years ago.” Of course, when an action or condition has subsisted since five years ago up to the present, the present perfect is called for, as in this sentence: “Clara and I
have been friends since we
met in a bookstore five years ago.”
2. “Stinson is a fantastic writer. He
has written ten very creative short stories
in the last year. One day, he’ll be as famous as Hemingway.”
I think the tense of the second sentence above is faulty. Since the action of writing the ten very creative short stories took place in a definite time in the past (“last year”), it shouldn’t be in the present perfect “has written” but in the simple past tense “wrote” instead, as follows: “He
wrote ten very creative short stories
last year.” This should be the case even if the writing of the stories was spread out over the entire year; the important consideration for establishing the tense here is that all of the writing is now over for those ten short stories.
That statement should therefore be corrected to read as follows:
“Stinson is a fantastic writer. He
wrote ten very creative short stories
last year. One day, he’ll be as famous as Hemingway.”
Alternatively, we can use the present perfect instead for that statement in a construction like the following:
“Stinson is a fantastic writer who
has written ten very creative short stories,
doing them last year over a period of only twelve months. One day, he’ll be as famous as Hemingway.”
The use of the present perfect is called for only if the action in the sentence has continued from some time in the past up to the present or at the moment of speaking. An expectation expressed in a separate sentence that action will or might continue in the future—in this particular case, the writing of more short stories to the point of the writer probably becoming “as famous as Hemingway”—has nothing or little to do with the determination of the tense in a preceding sentence. Generally, the tense to use in a particular sentence is independently determined on the basis of the verb describing the action, the time of its occurrence, and the duration of that action. So as not to confuse the reader, however, the good writer sees to it that the proper sequence of tenses in a narrative or exposition is scrupulously observed. (Click this link in the Forum for a discussion of
“How to use the normal sequence-of-tenses rule for reported speech.”)
3. “I
have not had this much fun since I was a kid.”
The sentence above is clearly in the negative form of the present perfect, describing a situation that has not happened from the time that the speaker was a child up to the present or the moment of speaking, when it finally
did happen. It may come as a surprise to some, but the operative verb phrase in that sentence is “have had fun,” with “not” and “this much” as its adverbial modifiers. The main verb in that phrase is actually “had,” which is the past participle of “have,” which then uses the auxiliary verb “have” to form the present perfect “have had.”
And in answer to your last question, yes, Sentence 3 here is in the present perfect for the same reason as Sentence 2—except that Sentence 3 is in the negative sense of the present perfect while Sentence 2 is in its usual positive sense.