一门 measure word 1 We will all have a course in ancient history this term. 这个学期我们都有一门古代史课程 2 A gun goes off every day to mark exactly one o'clock. 每天都有一门大炮开炮表示一点整。 一的 In Chinese there is no such statement

一门 means a course 一的 means nothing

I've never seen either of those words.

It's hard to say... I would say it is simply a preference. For instance, it is absolutely fine to use 个 to measure an idea, or a bulb. It is acceptable, but not so good, to use 个 to measure a TV set, a car, a people or a film. Instead, 台，辆，位，部 should be used respectively. Normally, people expect very young children use 个 to measure these objects. And it is unacceptable to use 个 to measure a horse, a fish, a book, or a piece of cloud.

because 门， 只， 条，本， 名， 头 etc are the special measure words but 个 is a general measure word thus measure words usage is determined traditionally, eg: 匹 for horses, but 棵 for trees