none of my friend left early .she kept none of his letters.我不明白为什么前面的“朋友”要用单数?

题目

none of my friend left early .she kept none of his letters.我不明白为什么前面的“朋友”要用单数?
复数可以么?none of my friends.还有一句话 it's none of your business.这里可以改成复数么?或者说这里应该做另外一种理解?希望能够得到清晰的答案.

答案

none of my friend 不正确,应为:none of my friends
又如:None of your friends like the film,did they?
你的朋友中没人喜欢这部电影,是吗
none of your business 没你的事；别管闲事
这是固定搭配,不能改成复数.