This discussion pertains to the BASIC form of the 6 English tenses (as opposed to progressive form):
The present perfect tense is a unique tense in that it is the only one of all six tenses which bridges time.
While the present tense (basic form) can express something that one does on a recurring basis--"I take the bus to school," for instance--the action is still present time since whenever the person takes the bus, the action is, indeed, at that time present.
*The simple past is just that--simple, isolated past action(s) that began and ended in the past. "Mary telephoned Jim last night."
The future expresses action or state for a time to come. "We will call you tomorrow."
The past perfect is an action begun and completed in the past before another past action. "I had just opened the door when the phone rang."
The future perfect expresses action or state to be completed in the future. "By this time next week we will have finished Unit Four."
However, **the present perfect expresses an action begun in some indefinite past time that is completed in the present, thus bridging the past with present time. "I have finished my research paper." [This paper was begun hours ago, perhaps days ago, and it is now completed.]
In contrast to this present perfect tense, if one wishes to assign a definite time to the completion, one should use the simple past:
"I finished my research at 2:00 a.m. in the morning and submitted it at my 8:00 a.m. class on Wednesday." Thus, this action is different from saying, "I have finished my research paper in time to submit it to my one o'clock class today."
In English we also use the present perfect to relate that something has started in the past and continues into the present. The simple past implies that the action is over and done with.
Here are a couple of examples:
The boys worked hard. (They are DONE working.)
The boys have worked hard. (And they are done OR are continuing to work hard.)
Jim ran in the rain. (And now he is done.)
Jim has run in the rain. (And will continue to do so).
In American English, at least, the present perfect structure refers to an unspecified time in the past. I can say "the bell has rung" and I could mean that it just rang or I can mean that it rang 5 minutes ago. If I use the simple past, I am referring to a specific time (even if I don't say when that time was). If I say "the bell rang" a specific time is implied. I might be saying, "as they were playing, the bell rang." Or I might mean "the bell rang just now."