Learning to Use the Present Perfect Tense
The function of the Present Perfect is complicated. As the use of "Present" in its name suggests, the Present Perfect links the past to the present. It focuses on actions, events or feelings which happened in the past, but which affect the present. We often talk about the usage of Present Perfect in the context of unfinished time.
The Core Characteristic is about making past actions relevant to the present. In the sentence, I have seen the Taj Mahal, the speaker is focusing on the experience of having seen the Taj Mahal, without making any reference to when this happened. The speaker is indicating they have gained something (and they are somehow different now) through the experience of seeing this wonderful sight. As the time reference is not considered important, the Reference Point (R) is an indefinite point of time in the past (Diagram 1):