It is sometimes the case that students will have problems understanding when to use the Present Perfect Simple‏‎ and when to use the Past Simple‏‎.

In general, the Past Simple is used for an action completed in the past. (Note that we would use the Past Continuous‏‎ if that action was of long duration, for example.) It’s often, but not always, used with a time adverbial.

Last year an earthquake struck Athens.

Napoleon fought many battles.

Effectively you can think of this action as being completed in the past and doesn’t extend into the present; in the examples above the earthquake is finished and Napoleon does not fight battles any more.

Contrast this with the Present Perfect Simple where the action happened in the past and there’s a likelihood that they will continue to happen now.

Athens has experienced many earthquakes.

Here we can understand that Athens experienced earthquakes in the past but there’s also a strong likelihood that it will experience earthquakes now and in the future as well.

The American army has fought many battles.

Here we can see that the American army fought battles in the past and they are likely to fight battles both now and in the future.