The teacher explains a new rule every week
the teacher is explaining a new rule now
the teacher has explained a new rule already
the teacher explained a new rule last month
the teacher has been explaining a new rule for an hour last Monday
the teacher had been explaining new rule by 5 o clock yesterday
the teacher will explain a new rule next month
the teacher will be explaining a new rule tomorrow at 10 o clock
the teacher will have been explained a new rule by the end of the next lesson