1)we could not go out because it had been raining hard since early morning
2)She has been teaching at our school for twenty years now. 
3)Ring me up as soon as you come home.
4)he began to write his essay at three o'clock
5)We have been helping our librarian to put the books in the
right order for already three days, but we have arranged only half the books.
 объясните,пожалуйста, почему именно так?