Present Perfect vs Past Simple

(I have finished  vs  I  finished)

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 2005.
        (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 refer to past events that have just happened or 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 1995 to 2002
    ..... before he moved to London.
    ..... a long time ago.

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