3、e:自…...以来1)since+时间点He has stayed heresince 5 o’clock.2)since+时间段+agoHe has stayed heresince 5 hours ago.3)since+从句(句子要用一般过去时态)She has taught Englishsince he came here.for:(长达)for+时间段Hehaskeptthebookfor2weeks.since+一段时间+ago=for+时间段;sincetwoweeksago=fortwoweeks;Ihavekeptthe