past actions which happened immediately one after the other
She sealed the letter, put a stamp on it and posted it.
past habit or state
He used to go/went to school on foot
complete action or event which happened at a stated past time
She called an hour ago. (When ? An hour ago.)
action which happened at a definite past time although the time is not mentioned. This action is not connected with the present.
Shakespeare wrote a lot of plays. (Shakespeare is now dead; he won't write again. - period of time now finished)