用 is 单音词,使用 is 双音词,but in the meaning ,there is no difference between 用and使用, for an example, 请用英语 is 请使用英语, in modern Chinese people like双音词better than 单音词, in the same time, Chinese language thinks more highly of rhythm, therefore , in order to attain a better rhythm, a Chinese will choose 用or使用 according to the context.