Our teacher explains a new rule every lesson.
She is explaining a new rule right now.
She has just expected a new rule.
She explained a new rule yesterday.
She was explaining a new rule from 2 till 3 o'clock yesterday.
She had explained a new rule by 12 o'clock.
She will explain a new rule tomorrow.
She will be explaining a new rule from 1 till 3 tomorrow.
She will have explained a new rule by that time tomorrow.
Present Perfect используется для обозначения действия, совершившемся в прошлом, и это связано с настоящим, наличием какого-либо результата или последствий. И важен сам факт совершения действия, а не время.
<span>a) Before examining the patient the doctor questions him thoroughly.
b) While listening to the patient's heart the doctor heard a soft systolic murmur.
c) When asked about the cause of a duodenal ulcer, the student couldn't answer.
d) On having received the results of analyses the doctor made the diagnosis.
e) If arrested in time almost any disease can be cured.</span>
Трилистник (Shamrock)
А вот почему - фиг знает :І