14.it's great to be back表示这个人是从其他地方回到这个城市的,并不一直住在这个城市,只是以前住过,所以要用过去时而不是完成时；
15.这句话的结构是：we believed引出一个长的句子（宾语从句）,然后在这个长的句子中,the time and hard work involved in completing such a important project这个部分是主语,are是谓语,所以这里involved in completing such a important project是一个后置定语,用来修饰the time and hard work,所以用involve的被动态involved.
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再答： ����˵�ɣ�the time and hard work involved in completing such a important project�Ǹ���ʣ��൱��i, you; ��are�Ǹ�be���ʣ���д�������ʱ������д�����д���ʵİɣ����ǽ����������ν��Ǿ��ӵĻ���ɲ��֡� ��the time and hard work involved in completing such a important project�������������չ����һ������ľ��Ӿ���the time and hard work are involved in completing such a important project�� ����Ҫ������һ�������Ҫ��be����ȥ����ʣ�µIJ��־���ԭ���ˡ�
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