咦(yí) - eh, oh (surprise) in Mandarin

咦 (yí) is the small noise you make when something does not add up: your keys are not where you left them, a friend turns up in the wrong city, a door you shut is standing open. It is surprise mixed with puzzlement, and the rising second tone is the whole performance.

It sits at the front of the sentence, followed by a comma, and the sentence itself is almost always a question. That is the pattern worth copying: 咦 plus 怎么 or 呢 or a 不是...吗 tag. The interjection sets the puzzlement and the question does the asking.

Do not confuse it with the neighbours. 哎 is closer to hey and often calls someone; 唉 is a sigh of resignation; 哦 is dawning understanding. And in writing, keep the character apart from 疑 (yí, to doubt) and the bare 一 (yī) once the tone marks come off in a listening drill.