She has been studying at the library since two o’clock. She has been studying at the library for two hours. They have been talking for an hour. They have been talking for a long time. They have been talking for ten minutes. I have been helping Mother since morning. I have been reading the book since the beginning of the holidays. They have been studying Italian since 2005. They have been studying Italian for a year. He has been working there since September.
since - с тех пор, с какого-то времени (он делает домашнее задание с трех часов, since three o'clock)
for - в течение какого-то времени (он делает домашнее задание 10 минут, for 10 min)
