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The tone that 一 is pronounced with is sometimes hard to predict. Is there a good general rule for when to use yi1 and when to use yi2/yi4?

The 不 tone change rule is fairly straightforward and easy to remember. 不 is underlyingly bu4 but changes to bu2 before another fourth tone, within a phrase. Maybe the 不-change rule can be blocked by a phrase boundary; I am not sure.

The 一 tone change rule (YTC) is much more confusing. 一 can be pronounced as yi1 or yi2 or yi4 according to both the surrounding environment and how 一 is being used.

For explaining the YTC, it almost seems like there are two separate words: one of which is always pronounced yi1 and the other is pronounced as yi4 or yi2 using the same rules as 不.

Wiktionary lists a bunch of terms with 一 pronounced as yi1, as yi2 and as yi4.

The yi2 and yi4 pronunciation appear to be in complementary distribution and never appear phrase-finally.

yi1 seems to appear in contexts where the corresponding two-related lexeme would use 二 instead of 两.

I am not sure that the distribution of 两 versus 二 is the exactly the same as yi2/yi4 versus yi1. Is there a better rule of thumb?

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The rule of thumb is the following.

When indexing it is yi1. For example

  • 一楼 (first floor)
  • 一声 (first tone)
  • 第一 (the first)
  • 一次世界大战 (World War I)
  • 一来 (firstly)

When counting （the same rule as 不）

  • it is yi4 when followed by 1, 2, 3 tone
    • 一呼百应
    • 一毛不拔
    • 一百
    • 一扫而光
  • it is yi2 when followed by 4 tone
    • 一蹴而就
    • 一气呵成

For others, mostly yi1. For example

  • 统一
  • 均一
  • 敬一丹（name）

As a comparison, 二 can be indexing or counting, but 两 is only counting.

PS: Most of the words in the Wikipedia list are right, but there are clear errors. For example "一yi4夫一yi4妻" appeared in 2 lists. And, it should be "一yi4朝一yi4夕". Don't fully trust it.

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  • Did you mean "第一次世界大战"? Also, to add, 一声 is yi1 when meaning the first tone in pronunciation, in other contexts it can be "counting" and pronounced yi4.
    – xngtng
    Oct 19, 2020 at 15:24
  • @zhantongz Yes you are right, I added the meaning there to reduce confusion. Depending on how you read it the meaning of these two words can change: "这一yi2次世界大战", and “叫了一yi4声” can be different from the meaning above.
    – River
    Oct 19, 2020 at 17:42
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    @GregoryNisbet Here is an example 两百元 and 二百元 both work, but 两元 is correct while 二元 is weird. It is not related to the tone of the following character, because 二人同行 and 两人同行 both work. Whether 二 can be used for counting is very complex and I cannot conclude a good rule for now. But the rule for 一 is rather clearcut.
    – River
    Oct 21, 2020 at 2:03
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    @GregoryNisbet only yi4bai3 is correct but yi1bai3 is not. This is different from 二 that 二百 and 两百 are both correct. That is why I say the rule for 一 is more clearcut than the rule for 二/两.
    – River
    Oct 21, 2020 at 14:12
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    Let me try to state it in a better way: If we only consider counting and indexing, yi2/4 is always counting and yi1 is always indexing. 两 is always counting, but 二 can be counting or indexing. However, when 二 can be used as counting is complex.
    – River
    Oct 21, 2020 at 14:19
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真的是个好问题，我原来总结过一些规律，但可能不靠谱。我感觉是根前后字（组合成的那个字）有关。 比如：

一二三四，读1声

一骑绝尘，读2声 （如果骑读作ji4的话）

一马当先，读4声

我发现，除了"一二三四"，如果"一"后面跟4声的词，"一"应该读2声，除了上面的例子还有诸如：

一箭双雕，一泄千里，一世英明……

如果"一"后面是1声、2声、或3声，"一"应该读4声，诸如：

一帆风顺、一尘不染、一鼓作气、一往无前……

也不知道准不准，欢迎提出反例。

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  • Older versions of the dictionary will say 一骑yi2ji4, but newer versions say yi4qi2, in PRC.
    – River
    Oct 19, 2020 at 2:42
  • @River，确实，如果读骑读ji4，一应该读2声，如果骑读作qi2，一应该读4声。
    – Zhang
    Oct 19, 2020 at 3:14

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