Simple Past
1. you know precisely when the action took place in the past.
   I saw her last week.  (when? last week)
 
2. a past habit which is now finished.
   She smoked a lot. (not anymore now)

3. in the second conditional.
   She would help him, if she knew he was in troubles.

4. a state or situation which started in the past and is now finished.
   I lived there for a long time.  (I am living somewhere else now.)

5. a finished state or activity and the period of time in which it has taken place is  finished.
   I wrote a letter this morning.  (the letter is finished and it is no longer morning)