Simple past tense vs. perfect past tense [duplicate]
Solution 1:
Let me use two different sentences to explain. 'I decided to get a degree in computer science when I was in university.' 'I have decided to reply to your question.'
The first, in the past tense, refers to an act that took place in the past and is not directly relevant to what's going on at this present time. If you were writing a history you would use the past tense a great deal.
The second, in the present perfect tense, refers to an act that has been completed and has an effect on the present time.