IT IS USED TO EXPRESS ACTIONS THAT TOOK PLACE AT SOME POINT OF TIME IN THE PAST AND ARE OVER NOW.
E.G.- SHE RECEIVED THIS LETTER YESTERDAY.
SINGULAR/PLURAL SUBJECTS + PAST FORM OF THE VERB
(* VERB+ED/D) - HAPPEN+ED= HAPPENED
(*THE VERB CHANGES ITSELF) - SEND(SIMPLE PRESENT)
SENT(SIMPLE PAST)