As mentioned in earlier posts, simple past tense is used to talk about an action completed in the past at a definite time or in a period of time that has now ended. The present perfect is also used for past actions but when the time is not mentioned or the action has its impact or results in the present. For example:

I came back last week.( definite time )

I’ve always lived in a houseboat.( I’m still living in a houseboat)

He last attended a party three months ago.( definite time )

He has not attended a party for three months.( period of time still continued )
Note that a conversation about a past event usually starts with a question and answer in the present perfect which becomes definite point of time for further conversation. Hence, rest of the conversation continues in the past tense form. For example:

Where have you been?

I’ve been to the science museum.

What did you see there?

I saw some great innovations and discoveries made by our great scientists.

Did you enjoy?

Oh, I loved it!

Similarly, we often find the present perfect being used to introduce an action in the newspaper or a news broadcast which is later continued in the simple past tense. For example:

Sixteen year old Kiara Nirghin from South Africa has won a scholarship at the annual Google Science Fair. She submitted a project aimed at alleviating severe drought conditions. She used orange peel to develop a cheaper super-absorbent material that helps soil retain water. ( an excerpt from The Times of India, student edition, October 4’16 )

Pakistan’s former PM Pervez Musharraf has blamed the Nawaz Sharif govt’s “wrong policies” for its global isolation. He also criticised India for its “accusations” against Pakistan. ( an excerpt from The Times of India, student edition, October 4’16 )