How to Say "Which One" in Chinese

哪一个

nǎ yí gè

The phrase "which one" in Chinese is 哪一个 (nǎ yí gè).

哪一个 is used to ask for a specific choice among a group of items.

What does it mean word for word?

The phrase is composed of 哪 (which), 一 (one) and 个 (a general classifier or measure word).

哪一个 is more grammatically complete and is often used when emphasizing the selection of a single specific item from a group.

nǎ yí gè

When using 哪一个 to ask about people, you must replace the general classifier 个 with 位 to be polite. Asking 哪一位 sounds much more respectful than 哪一个 when referring to a person.
The classifier 个 used in 哪一个 is the most common measure word, but it is often considered too informal or 'low level' for elegant writing. In professional contexts, you would replace it with a specific classifier suited to the object.
The character 哪 contains the 'mouth' radical on the left, a common feature in many Chinese question words used to signal an inquiry.

我们觉得很难决定买哪一个。
We find it difficult to decide which one to buy.
This sentence is useful for describing the difficulty of deciding which one to buy with 哪一个.
Tatoeba #1415814 by sadhen, translation #37905 by AlanF_US
你见到的是两兄弟中的哪一个？
Which one of the two brothers did you see?
This sentence is useful for asking which one of two brothers was seen using 哪一个.
Tatoeba #1746064 by sadhen, translation #5618209 by adamtrousers
哪一个你喜欢一点？
Which one do you prefer?
This sentence is useful for asking which one a person prefers using 哪一个.
Tatoeba #400226 by sysko, translation #687708 by lukaszpp
这里有两本书，哪一个是你的？
There are two books here, which one is yours?
这两个电影院，哪边更近？
Between these two cinemas, which one is closer?
这两部电影，哪个好看？
Between these two movies, which one is better?
3 more sentences on Tatoeba

At a clothing store, choosing a shirt.

志强 Zhiqiang

你想要哪一个？
Nǐ xiǎng yào nǎ yī gè?
Which one do you want?
佳怡 Jiayi

我想买这件蓝色的。
Wǒ xiǎng mǎi zhè jiàn lán sè de.
I want to buy this blue one.
志强 Zhiqiang

这件很适合你。
Zhè jiàn hěn shì hé nǐ.
This one suits you well.
佳怡 Jiayi

谢谢，那我就拿这件吧。
Xièxiè, nà wǒ jiù ná zhè jiàn ba.
Thanks, then I will take this one.

Sources

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