____has been said before, it is not easy to learn a foreign language.

_______ difficult and challenging the task may be, we must finish it on time, because there is a possibility _______our work will go down in history.

_____ a pity that I forgot to bring your dictionary.

I would appreciate it ________ you call back this afternoon for the doctor's appointment.

My sister isn’t in. ________ she comes back, I’ll let her call you back.