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