It is used to express an action which started and finished in the past:
I got up at 8 yesterday.
she bought a new house 5 days ago.
I met with my best friend last week.
It is used to express a sequence of actions which happen one after another:
we drank coffee, went to the movies, and returned home.
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ATTENTION!
IF YOU USE WHEN IN A PAST SIMPLE SENTENCE IN WHICH BOTH CLAUSES CONTAIN VERBS IN THE PAST SIMPLE THE ACTION IN THE WHEN CLAUSE HAPPENS FIRST.
WHEN I WOKE UP, I WALKED MY DOG.
FIRST I WOKE UP AND THEN I WALKED MY DOG
*DON'T FORGET ABOUT THE COMMA WHICH SEPARATES TWO CLAUSES