1.He has left since yesterday .是错误的.
leave 是短暂性动词,不能和一段时间连用.since +过去的时间点 表示一段时间.
应该改为 He has been away since yesterday . be away 是延续性动词.
2.since yesterday / 6.30 、/ 2010 是可以的,它们都表示一段时间,不是时间点.
3.He has left since two days ago.是错误的,和第一句属于同一类型.
two days ago 是表示过去的时间点,而since two days ago “自从两天以前”,表示时间段.
正确的说法是：He has been away since two days ago.
4. He has been away since two days ago 是正确的.
5.这种说法完全正确.
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