Miss Gao has been away for an hour.
Miss Gao has been away since an hour ago.
It is an hour since Miss Gao left.
一二句是现在完成时 因为 leave 是短暂性动词 不能与since 和 for 连用 所以 用 be away
第三句是固定结构 以 It is 开头 since后面还是过去时
Miss Gao has been away for an hour.
Miss Gao has been away since an hour ago.
It is an hour since Miss Gao left.
一二句是现在完成时 因为 leave 是短暂性动词 不能与since 和 for 连用 所以 用 be away
第三句是固定结构 以 It is 开头 since后面还是过去时