a pile of意思為“一堆……”根據慣用法表示單數含義。
piles of意思是“成堆的……”,後面謂語要用複數相應的形式。
1. a pile of後接名詞複數或不可數名詞（a pile of既可以修飾名詞，也可以修飾不可數名詞），根據慣用法一般視為單數含義，請看：
There in front of me was a great pile of old cans.
我的面前是一大堆舊罐頭。
There was a pile of clean laundry on her bed. 她床上有一堆剛洗好的衣物。
A pile of ironing is hardly inspiring. 一堆需要熨燙的衣服無法讓人提起興緻。
Heaped up in one corner was a pile of junk. 堆在一角的是一堆廢物。
There was a great big pile of books on the table.
桌上有很大一摞書。
There was a (neat) pile of books on the desk. 桌子上有一堆（整齊的）書本。
On the table beside an empty plate was a pile of books.
桌上空盤子的旁邊是一堆書。
On the table was a pile of books and a scatter of papers.
桌子上放着一摞書和零落的幾份報紙。
Under a wide shelf that holds coffee jars stands a pile of magazines
放咖啡罐的一個寬架子下方有一摞雜志。
I was looking for my handbag, which was buried under a pile of old newspapers.
我正在找我那被埋在一堆舊報紙下面的手提包。
If we thought Dad's pile of chicken manure was bad, this was 10 times worse.
如果我們認為爸爸那堆雞糞很糟糕的話，這就是那個的十倍。
聯考真題- 2015 四川
特例用複數的例子：
A pile of presents were heaped under the Christmas tree.
聖誕樹下有一大堆禮物。(摘自Easier English Student Dictionary)
2. piles of（後面可加可數名詞，也可以不可數名詞；表複數概念，作主語時謂語一律用複數形式。）
一堆堆的，成堆的；一大團；一大批
I have got piles of work to do.
我有大量工作要做。
There were neat piles of kindling wood against the wall.
牆邊整齊地放着幾堆引火柴。
There are piles of marvellous stones in the garden.
園中奇石磊磊。
There were piles of litter in the streets.
街道上垃圾成堆。
The bookshelfves were full and there were piles of books all over the floor.
書架上裝滿了書，地上也一堆一堆的書。