1. Time expressions such as : this morning, this week, can be used with either past simple or present perfect verbs. If we think of this morning as a past, then we use the past simple. If wet think of this morning as a time period which includes the present moment, then we use the present perfect.
   1. Compare:
      1. I didn't shave this morning.
      2. I haven't shaved this morning.
2. In a sentence which includes a time clause with since, we generally prefer a past simple verb in the time clause and a present perfect verb in the main clause. The time clause refers to a particular point in the past:
   1. Since Mr Hassan became president, both taxes and unemployment have increased,
   2. She hasn't been able to play tennis since she broke her arm.
3. With time clauses introduced by after, when, until, the past simple refers to past, completed events and the present perfect refers to future events.
   1. Compare:
      1. After she left the hospital, she had a long holiday
      2. After Dominic has left school, he will be spending six months in India