Mary was looking at her in the mirror 是“玛丽正在照镜子”吗？
这位网友的翻译有问题，错误就出在代词用错了，应该使用“反身代词”(也叫“自身代词”)herself，而不是“宾格代词”her。从语法和逻辑两方面考虑，如果动作的施事者(发出者)又充当了动作的接受者，那么必须使用反身代词，不能使用宾格代词。因为在同一个句子中，宾格代词不表示主语本人，而是另外一个人，反身代词和宾格代词的差别就在于此。Mary was looking at her in the mirror.这个句子本身不错，但意思是：玛丽正在镜子里看她(网友本人)。
再举几个例子，请比较：
She was dressing herself. 她正在穿衣。(给自己穿)
She was dressing her. 她正在给她穿衣。(给别人穿，可能是她女儿)
John should not praise himself. 约翰不应该自吹自擂。
John should not praise him. 约翰不应该表扬他。(表扬别人)
Alice often speaks to herself. 爱丽丝经常自言自语。
Alice often speaks to her. 爱丽丝经常跟她讲话。(跟别人讲话)
The young man taught himself physics. 那位年轻人自学物理。
The young man taught him physics. 那位年轻人教他物理。(教别人)
She should learn to take care of herself. 她应该学会自己照顾自己。
She should learn to take care of her. 她应该学会照顾她。(可能是自己的妹妹)
Tom bought a new bicycle for himself. 汤姆给自己买了一辆新自行车。
Tom bought a new bicycle for him. 汤姆给他买了一辆新自行车。(给别人买自行车)
下面是一些常见的含有反身代词的固定词组，请熟记于心：
abandon oneself to 沉迷于，放纵于 accustom oneself to 习惯于
amuse oneself 自我消遣 come to oneself 恢复知觉
be oneself 状态正常 be not oneself 身体不舒服
behave oneself 举止得体 lose oneself 迷路
dress oneself 穿衣 seat oneself 坐下
devote oneself to 致力于 help oneself to 随便吃，随便用
enjoy oneself 过得快活 express oneself 表达自己的想法
feel oneself 感觉正常 teach oneself 自学
think to oneself 心里想 make oneself at home 不要拘束
