This westward movement, on the one hand, increased the agony of displaced Native Americans, and on the other, opened up new territories for settlement, pushingthe frontier to the Rockies, thento the waters of the Pacific.
请问：
为何 push 用目前分词（逻辑主语是？）而不是过去时？
to the waters 前是否省略了什么？
This westward movement, on the one hand, increased the agony of displaced Native Americans, and on the other, opened up new territories for settlement, pushingthe frontier to the Rockies, thento the waters of the Pacific.
请问：
为何 push 用目前分词（逻辑主语是？）而不是过去时？
【答】目前分词短语pushingthe frontier to the Rockies, thento the waters of the Pacific，作结果状语，其逻辑主语是movement（西进运动）。
假若用pushed，那样跟increased，opened up 就是并列。但，前面increased和opened up已经并列了，只能用分词形式了。注意红色部分：This westward movement, on the one hand, increased the agony of displaced Native Americans, and on the other, opened up new territories for settlement。
to the waters 前是否省略了什么？
【答】我感觉，在then 之前应加and，由于then 是时间副词，不是连词。如此使两个介词短语并列：to the Rockies, and thento the waters of the Pacific。
【翻译】这项西进运动，一方面增加了流离失所的土著美国人的痛苦，其次为殖民开辟了新的领土，（结果）把边疆扩展到落基山脉，然后到了太平洋水域。
