
two days before指过去某个时间点的两天以前;主句需要使用完成时或者在含有宾语从句的间接引语中;two days ago指从现在算起两天前;主句需要使用过去时。eg.I visited him three days ago,but he had gone to Shanghai a week before.我三天前去访问他,但他已于那时的一周前到上海去了.

two days before指过去某个时间点的两天以前;主句需要使用完成时或者在含有宾语从句的间接引语中;two days ago指从现在算起两天前;主句需要使用过去时。eg.I visited him three days ago,but he had gone to Shanghai a week before.我三天前去访问他,但他已于那时的一周前到上海去了.