- A bag of potatoes was discovered in his car. (他的車上被發現一袋馬鈴薯) (正)
- A bag of potatoes were discovered in his car. (誤)
解說:「A bag of + 複數可數名詞」若當句子的主詞,其真正的主詞為 bag,後面須接單數動詞,亦即句子的主動詞須與 bag 一致,而不是跟複數可數名詞一致。介系詞片語「of + 複數可數名詞」係當形容詞用,修飾 bag,對動詞沒有影響。除 a bag of 外,a box of, a pack of 等片語的用法亦同。