In the dialogue, Mary is talking about the same thing. However, grammars permit and require her to use different tenses -- Present Perfect and Simple Past. It is true that the meanings of her sentences are not exactly the same, but we will agree Mary relates the same thing, and most of all, the same thing of the same time. We cannot argue that, before Mary has mentioned the time, the Present Perfect action has no time at all. It is also absurd to suggest that Mary didn't know the time of the Present Perfect action. She knows. But grammars permit her to use Present Perfect. Later when she adds the time, grammars require her to use Simple Past. We can hardly imagine that one will deny the truth and argue, "There are two happenings here. One is Mary has seen Paul; another, Mary saw Paul last Saturday." This argument is impossible. Furthermore, the fact that Mary has added 'Last Saturday' and not repeated the rest, indicates that the rest is same as above, though in different tenses. The two tenses are different in format, but not in meaning, nor in time. Therefore, it is small wonder that we have been trying for so long to tell apart the two tenses, and getting nothing in return. As the dialogue shows, there is the process of tense-changing: if we tell of the time, we have to change the format of tense.Mary: "I have seen Paul."
John: "When?"
Mary: "Last Saturday." (Which has to mean "I saw him last Saturday".)
And yet, the 'Present Perfect to Simple Past' conversion above is not unique among English tenses. I have found out the same logic applies also to Simple Present. Let's look into another similar conversation:
As in the previous dialogue, now grammars permit and require Mary to use both Simple Present and Present Perfect to describe the same thing of the same time. Here for example, it is the same work, logically of the same time. It is absurd to suggest that, in Simple Present, Mary doesn't know the time. Again, specifying the time of work does not change the work at all, but the tense. The fact that Mary has added 'since 1987' and not repeated the rest, indicates that the rest is same as above, though in different tenses. Because of the mention of the time, the 'Simple Present to Present Perfect' conversion is again mandatory. Having found the common point in the two dialogues, therefore, I am now convinced there is the tense-changing process.Mary: "I work in the airport."
John: "Really?"
Mary: "Yes, since 1987." (Which has to mean "I have worked in the airport since 1987".)
In the past we have always tried to explain that different tenses are used to tell different indications. This has been not wholly correct. We understand now we may sometimes use different tenses to say the same thing of the same time. Taken separately, the conversions seem confusing: how can two tenses be the same? However, after we have truly recognized and displayed all these 'confusions', the relations of the three tenses are finally revealed, in an obvious pattern. The tense-changing process is indeed very simple: inserting a Definite Past Time Adverbial (DPTA) into an action will change its tense. The whole process can be rendered by the following four simple rules:
(a) Simple Present action indicates a present action (= incompletion):
Ex: I live in Hong Kong.
(b) Present Perfect action indicates a past action (= completion):
Ex: I have lived in Japan.
BUT: If we mention a definite past time, tenses have to be changed:
(c) Present Perfect action indicates a present action (=incompletion =a):
Ex: I have lived in Hong Kong in the past three years.
(d) Simple Past action indicates a past action (=completion =b):
Ex: I lived in Japan five years ago.
These four rules seem simple, but they cover all kinds of sentences in these three tenses. They describe the relations and interactions between these tenses.
Present Perfect is difficult only when we see it separately.