Present Perfect or Past Simple?

PRESENT PERFECT vs PAST SIMPLE

(ex: I have finished vs I finished)
When do we use them?

When do we use the Present Perfect and when do we use the Past Simple?

PRESENT PERFECT

The present perfect tense is used :

To talk about an action which started in the past and continues today :

For and since are used to express duration.
- I have lived here for 10 years. I have lived here since 2008.
  (I arrived 10 years ago and I am still here.)

To refer to past events in an unfinished period of time.
- I have written two letters today.
  (Two letters are written but today is not finished.)

To talk about recently completed actions with 'just'.
- "I've just finished reading the book you lent me."

To refer to past events that have happened recently or have just been announced :
- There has been a plane crash near the coast.
  (We know the event took place but we don't know when.)

To talk about a past action with a result in the present:
- I've broken my arm. I can't drive (= because my arm is broken now.)

To talk and ask about experiences or accomplishments up to the time of speaking :
- "I'm a writer. I've written 9 books."
- "Have you ever written a biography?" "No, never."

PAST SIMPLE

The Past Simple is used:

When the period of time is finished.
- I wrote two letters yesterday. (Yesterday is finished).
When the time is mentioned, either a precise time in the past, or a time expression which clearly situates the event in the past.
- John lived in the country
  ..... until the age of 6.
  ..... when he was young.
  ..... from 2004 to 2010
  ..... before he moved to London.
  ..... a long time ago.

As a general rule, if you can answer the question "when?", use the Past Simple.