▲关于这个问题，我与前面两位老师的观点稍有不同：“a pile of+复数名词”作主语时，谓语动词可以用单数（视为整体），也可以用复数（考虑其个体），但以用单数为普通。如：

——用单数的例子：

There was a pile of books on the table. 桌上有一堆书。

On the table was a pile of books and a scatter of papers.  桌子上放着一摞书和零落的几份报纸。

——用复数的例子：

A pile of presents were heaped under the Christmas tree. 圣诞树下有一大堆礼物。(摘自Easier English Student Dictionary)

▲当然，如果 a pile of 后接不可数名词，它作主语时，谓语动词只能用单数。如：

There was a pile of clean laundry on her bed. 她床上有一堆刚洗好的衣物。

A pile of ironing is hardly inspiring. 一堆需要熨烫的衣服无法让人提起兴致。

Heaped up in one corner was a pile of junk.  堆在一角的是一堆废物。