Today's Idiom  

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.