ear单数用an。因为ear是元音字母开头的，元音字母开头的单词前面都用an。an ear的意思是耳朵，一只耳朵，一个耳朵。an ear的例句：He suddenly cocked an ear and listened（他突然竖起耳朵仔细倾听）。

an ear的例句：I was only listening to the radio with half an ear（我只是顺便听着收音机）；An ear for foreign languages is advantageous（能够听懂外语是种优势）。