army 表军队的总称，与navy(海军)、air force(空军)并列时指陆军。
troops 常用复数，多指陆军，着重构成军队的士兵成员，有时也指骑兵部队。
forces 语体较正式，多用复数形式，既可指包括陆、海、空在内的一个国家的整个武装力量，也可指某一兵种。
参考例句
1. The army is under his direct command . 陆军是在他的直接指挥之下。
2. You need a quite big army to raid an oasis with profit. 但你需要一队相当大的军队用利润突袭一个绿洲。
3. They want officers in the Mexican Army. We could be so happy there. 墨西哥的军队需要军官，我们可以在那里过得很快乐的。
英英解释
n.
1. a permanent organization of the military land forces of a nation or state
2. a large number of people united for some specific purpose
3. the army of the United States of America; the agency that organizes and trains soldiers for land war
