核心问题
章振邦等编《新编英语语法》(下)(p.696)在讲限制性关系分句时举了一个正面例句：The reason why I was alone in the mountains is because I had a difficulty with my guide. 请问这个例句是否正确？
词典解答
语言学家们认为 The reason is because 犯了语义重复的毛病, 是应当避免的。例如,
CGEL(§ 15.46)把下列句子看作是典型错误：
The reason (that) they didn't go to see the house is because they prefer to live in an apartment. 他们没去看房子的原因是他们宁可住在公寓里。
并指出，只有把 because 改为 that，上面的句子才是可以接受的。
F. T. Wood 则用下列句子来说明 The reason is because 是一种常犯的语言错误：
The reason for the delay is because we have had difficulty in obtaining materials. 耽搁的原因是因为我们在获取原材料方面遇到了困难。
并建议改为 The delay has arisen because...
希拉里在其回忆录《亲历历史》中也犯有类似的错误：
The official reason I am here today is because I have had that responsibility. (Hillary, Living History) 我今天来这里的原因是我负有此项责任。