The rule can be simple for teaching purpose:
所有跟贴·加跟贴·新语丝读书论坛
送交者: meiyou 于 2006-11-10, 21:47:57:
回答: 奇怪的汉语: 量词太复杂了 由 alexdongli 于 2006-11-10, 21:20:47:
If 量词 can be omitted without causing ambiguity(for exmaple: 一个月 vs 一月 is not in this category), the shorter form is always allowed - and you can say prefered too.
所有跟贴:
加跟贴