a quantity of+可数名词复数、谓语动词用复数；a quantity of+不可数名词、谓语动词用单数。即修饰可数或不可数名词均可，作主语时采取就近一致的原则，其谓语动词通常与短语中of后面的名词的数保持一致。
a quantity of谓语用单数还是复数
a quantity of+可数名词复数、谓语动词用复数；a quantity of+不可数名词、谓语动词用单数。
若单独用quantity作主语，谓语动词用单数。
如：It is used to emphasize that the quantity is very large。
它用于强调数量很大。若该结构变成(large)quantities of…这样的形式，则当它用作主语时，其谓语动词一律用复数。
如：Huge quantities of oil were spilling into the sea。大量的石油渗到海里。
quantities of和a quantity of的区别
quantities of和a quantity of都可以修饰可数名词和不可数名词，区别在于：
quantities of在修饰不可数名词，并且和后面的名词一起做主语时，谓语由它决定，也就是用复数。
a quantity of修饰不可数名词，并且和名词一起做主语时，谓语动词由后面的名词决定，也就是用单数。
例如：Quantities of energy are saved today. (今天大量的能源被节约)
A quantities of energy are saved today.
Quantities of cash have been withdrawn from the bank.
(大量的现金已经被从银行取出)
Aquantity of ready-use 35 mm ammunition in clips is also carried.
一批准备使用35毫米弹药在剪辑，也获得通过。
其他时候俩者可互换。
The milk carton, in turn, has a quantity of milk inside it.
=The milk carton, in turn, has quantities of milk inside it
这盒牛奶，再进一步，它里面牛奶有一个量。