Поделитесь своими знаниями, ответьте на вопрос:
Write the correct form: 1) listen! someone (to knock) at the door. 2) don't go into the classroom! the students (to write) a dictation there. 3) they (to discuss) it since five o'clock. 4) where is petrov ? -he (to work) in the library. -he (to work) long ? -yes, he (to work) since morning.