The following is an excerpt from an explanation on ego4u.com: (English Grammar Online)
In British English, the use of Simple Past and Present Perfect is quite strict. As soon as a time expression in the past is given, you have to use Simple Past. If there are no signal words, you must decide if we just talk about an action in the past or if its consequence in the present is important.
In American English, you can normally use Simple Past instead of Present Perfect. We cannot accept this in our exercises, however, as this would lead to confusions amongst those who have to learn the differences.
Emphasis on action
- I bought a new bike. (just telling what I did in the past.)
Emphasis on result
- I have bought a new bike. (With this sentence I actually want to
express that I have a new bike now.)