a number of后只能接可数名词,如果在a后面再加great就变a great number of就等于many(many修饰可数名词）另外还有一个the number of,它们的区别就是,the number of后面加单数动词,举个例句,The number of students in our class is fifty(注意这个is是单数动词）,a number of后面加复数动词,举个例句,A number of students in our class are from Shanghai(注意这个are是复数动词）,a lot of 既可接可数又可接不可数名词,另外a lot of 就等于lots of