1.army是集体名词,基本意思是“军队”(尤指陆军),与air force, navy相对。引申可指“一大群”。
2.army用作主语时,其谓语动词可以是单数形式(强调整体),也可以是复数形式(强调个体)。
3.an army of意思是“大群,大批”,多接复数名词,但其谓语动词一般为单数形式。

The people's army fights for the people.人民军队为人民而战。
Only after you have reached eighteen can you join the army.你只有年满18岁之后才可以参军。
The army is in a high state of readiness.军队处于高度戒备状态。
The army was beaten.部队打了败仗。
Before long he rebuilt a strong army and defeated the enemy.不久之后,他重新建立了一支强大的军队并打败了敌人。
A strong army and a strong airforce are indispensable to the defence of a land locked country.一个强有力的陆军和一个强有力的空军对于保卫一个四周皆陆地的国家来说是必不可少的。
The army are clearing up quite a bit of land.军队在整治大片的土地。
An army of workmen was brought in to build the stadium.一大群工人被招来建筑运动场。
An army of teenagers came to the concert to hear the famous singing group.一大群青年人去听著名演唱团的音乐会。
An army of buyers haggles furiously with an army of salesmen.一大群买主在和卖主拼命地讨价还价。

army, forces, military, troops
这组词的共同意思是“军队”“部队”。其区别是：army泛指“武装部队”时,语气比较正式、庄重,它常指“陆军”; military尤指“陆军”,包括全体战斗人员及工作人员,代表军方区别于政府及其他各部门;forces是集合名词,包括国家的各兵种的所有人员及武器装备; troops较通俗,指部队的全体成员,尤指“陆军部队”,有时专指包括各个较低级军衔在内的队伍,以区别于上级指挥官。例如：
They are determined to build up a modern army and navy.他们决心创建一支现代化的陆军和海军。
The military took control of the government in that country.在那个国家,军方控制了政府。
The commander concentrated a powerful force on the left flank.司令员把强大兵力集中在左翼。
The navy is one of the armed forces.海军是武装部队之一。
They massed well-equipped troops on the frontier.他们在边境上集结了装备精良的军队。
The troops were drawn up ready for inspection.部队整好队准备接受检阅。