shéi?
Who?
这是什么?
What is this?
This is a question: 这 (zhè) means 'this', 是 (shì) means 'is', 什么 (shénme) means 'what'.
这是什么东西?
zhè shì shénme dōngxi?
What is this thing? — More colloquial, with 'thing' added.
那是什么?
nà shì shénme?
What is that? — Used for something farther away.
这是什么?
zhè shì shénme?
What is this?
这是一个手机。
zhè shì yī gè shǒujī.
This is a mobile phone.
Appropriate
Avoid when
You say '这是什么?' (zhè shì shénme?).
It is pronounced 'zhè shì shénme?' with the fourth tone on 'zhè', fourth tone on 'shì', second tone on 'shén', and neutral tone on 'me'.
Use it when you see something unfamiliar and want to know what it is. It is a basic, neutral question suitable for everyday situations.
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