Difference between Simle Past and Present Perfect

Simple Past:
I read the article yesterday.
--specific time, task completed.

Examples:
I worked hard "yesterday/last week/last month"

Questioning:
When did you move to USA? --- exact time


Present Perfect:
I have read the article.
--no specific time, task completed sometime in past.

Examples:
I have worked hard this "week/semester"
I have worked hard since "6 am/January"

Questioning:
How long have you been in USA? --- duration of time

Usage of Past Simple and Past Perfect

8:00 -- wake up
8:15 -- eat breakfast

After I had woken up, I ate breakfast.
Past Perfect + Simple Past

5:15 -- have a bath
5:30 -- watch TV

After I had had a bath, I watched TV.

English Grammar

Sites for English Grammar.

http://web2.uvcs.uvic.ca/courses/elc/studyzone/