There is some degree of overlap between the simple past and the present perfect however there are important differences. When a specific past time is specified, the simple past should be used.

When the time is unspecified, either the present perfect or the simple past can be used.

The simple past expresses more finality than the present perfect.

In the sentences above, the first sentence leaves open the possibility that John continues to teach mathematics. The second implies that John no longer teaches mathematics. Maybe he retired, changed careers, or passed away.

The same is true for the sentences above. The second sentence expresses more finality. Maybe the restaurant burned downed, maybe Steve and Carol moved somewhere else or swore never to eat at the restaurant again.

If something began in the past and continues until the moment of speech, the present perfect is used.

The sentences below imply that the activity was begun in the past and has already ended.

When since is the sentence, the present progressive is always used.