一、是没有复数的形式；(milk, milk, milk) 　　二、是作主语,谓语须用单；(The water is clean. The air is fresh. ) 　　三、是其前不用a和an；(The work is hard to do. ) 　　四、是修饰不用基数词； 　　五、是可用much、some、any、a lot of等修饰；(I want to drink some wa