Countable Nouns & Uncountable Nouns

In English, most countable nouns [C] have a beginning and an end.

Uncountable nouns usually have no clear beginning or end, and no shape.

Some nouns have two or more meanings. [C] has one meaning, and [U] has another meaning.

Learner's dictionaries show this very clearly. The Cambridge Learner's Dictionary, for example:

hair [U] = 毛，hair [C] = 頭髮

paper [U] = 紙張, paper [C] = ,（一份）報紙

room [C] = 房間, room [U] = 空間