5、当作介词或连词使用。ago不具有这一功能。如:Theywillcomebackbeforesixo’clock.Itishopedthatthiswillbefinishedbeforetheyear1995.希望这件工作将在1995年以前完成。Exercises1、Mr Smith explained that Tom had told him about his past ____. A.three weeks ago B.before three weeks C.sincethreeweeksD.three