1 a bag of; a sack of

● 这两个词组介词均意为“一袋”，都以容积表数量单位。

● a bag of中的bag比a sack of中的sack小，表布、纸、皮革等材料做成的袋子，如：He brought home five bags of groceries.（他带回五袋日用食品。）Peter, come in, here is a bag of candies for you.（过来，彼得，给你一袋糖果。）I have two bags of flour to sell.（我有两袋面粉要卖。）My sister walked with a bag of toys across the garden in the grey light of the dawn.（我妹妹拿着一袋玩具，在晨曦中穿过了花园。）

● a sack of中的sack比bag大，多为牛皮纸或麻布等材料做成，也译为“一包”等，以示较“一袋”大。如：She bought two sacks of flour.（她买了两袋面粉。）We burned four sacks of coal.（我们烧了四袋煤。）He is strong enough to carry a sack of rice.（他力气大，背得起一包大米。）Her brother bought a sack of potatoes in the shop.（她弟弟在商店买了一袋马铃薯。）

● 有时，它们可以互换。