get a letter from sb.为什么letter不是复数形势,mail letters却是复数?

get a letter from sb.为什么letter不是复数形势,mail letters却是复数? hear from sb receive（此单词可能有误） a letter from sb get a letter letter复数? letter复数是什么? receive a letter from sb 是什么意思? this is a letter改为复数句 l get a letter from my penfriend get a letter from的同义词 receive from=hear from=get a letter from business letter 怎么变复数 letter的复数形式是什么 get a letter from,received a letter ,heard from his invition