诶(ēi) - hey, oi; oh, huh (surprise or getting attention) in Mandarin

诶 (ēi) is the noise you make to catch someone's attention or to register a small surprise. It sits at the front of a sentence, never inside one, and it is written with the speech radical because that is all it is: a sound with a job.

This character carries four readings and they mean different things. First tone ēi is hey. Second tone éi is what, said when something does not add up. Third tone ěi pushes back on what was just said. Fourth tone èi agrees, the spoken equivalent of a nod. Speakers use all four without thinking about it.

The trap is politeness. 诶 is casual to the point of blunt, fine with friends and family and wrong with a stranger, a teacher or anyone senior to you. Use 请问 to open a question with a stranger, and save 诶 for people who already like you.