How do you differentiate between the use of present perfect tense and simple past tense in English grammar?
Let's look at the following two sentences:
I lost my keys.
I have lost my keys.
First sentence (in Past Simple) informs only about the past event (loosing the keys) but we don't know if the speaker has the keys now. He might have found the keys later.
The second sentence emphasizes the connection between the past event and the present situation.
I have lost my keys. means that I don't have them now.
Another example of using Present Perfect can be a dialogue:
-Do you want anything to eat?
-No, I have just had a lunch.
The most important information is not that someone had a lunch, but that he/she is not hungry because he/she has had a lunch recently.