英语中come on是一个形容词，意思是共同的，共有的，普通的。common不是名词，所以不存在可数和不可数这种说法。

Common sense常识。

例如

It is common for the people to havePrivate carsin China.

Nowadaysalmost every student has a mobile phone mobile phone which is a common phenomenon.

Common sense is playing an important part in our daily life.

common是形容词，不是名词，不存在可数不可数。