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.
The PAST PERFECT TENSE indicates that an action was completed (finished or “perfected”) at some point in the past before something else happened. This tense is formed with the past tense form of “to have” (HAD) plus the past participle of the verb (which can be either regular or irregular in form):
I had walked two miles by lunchtime.
I had run three other marathons before entering the Boston Marathon .
The PAST PERFECT PROGRESSIVE TENSE indicates a continuous action that was completed at some point in the past. This tense is formed with the modal “HAD” plus “BEEN,” plus the present participle of the verb (with an -ing ending): “I had been working in the garden all morning. George had been painting his house for weeks, but he finally gave up.”