The PAST TENSE indicates that an action is in the past relative to the speaker or writer.
when the time period has finished: "We went to Chicago last Christmas."
when the time period is definite: "We visited Mom last week."
with for, when the action is finished: "I worked with the FBI for two months."
Regular verbs use the verb's base form (scream, work) plus the -ed ending (screamed, worked). Irregular verbs alter their form in some other way (slept, drank, drove).

Students for whom English is a second language sometimes (quite understandably) have trouble distinguishing between the Simple Past and the Present Perfect tenses. There is more information about the difference between these two tenses available under the Present Perfect description.