This is pretty basic Chinese — how to say the numbers “one” and “two” — but I’ve not yet found a grammar book that explains the rules in a clear and precise way, so I’ve decided to do it myself.
We’ll start with “two,” since the distinction between the two forms of the number, 二 (èr) and 兩 (liǎng), is more obvious. Because two different characters with two completely different pronunciations are used, a few months’ exposure to spoken or written Chinese is usually enough to give one an intuitive feel for when to use which one — which is good, because the textbooks (and native speakers, if you ask them) are terrible at explaining it. Here’s a typical example from Practical Audio-Visual Chinese, published by 國立台灣師範大學國語教學中心:
Comparison of 二 and 兩: 二 is usually translated as “two”, and can be used alone. In counting, numbers with two or more digits that end with the number 2 use the character 二, not 兩. Such as 十二, 二十二. 兩 is a bound form, i.e. it can never be used alone. It must always be followed by a measure word.
This is seriously misleading in several ways. Below are the real rules, as I’ve discovered them using the linguistic field methods I learned back in college:
For the number “two” itself:
- If it’s a cardinal number used without a measure word — as in counting, in numbered lists, etc. — always use 二. “One, two, three” is 一 、二 、三.
- If it’s a cardinal number used with a measure word (such as 個), always use 兩. “Two apples” is 兩個蘋果.
- If it’s an ordinal number, always use 二, regardless of whether or not there is a measure word. “The second apple” is 第二個蘋果.
- In compound words, four-character idioms, etc., there don’t seem to be any hard-and-fast rules — but in general, 兩 seems to carry the meaning “both” (i.e., both members of an understood set of two), whereas 二 is “two” in a more general sense. Here are a few examples:
- 獨一無二 (“unique”; lit. “one alone, no two”), 二手貨 (“secondhand goods”), 一石二鳥 (“kill two birds with one stone”), 禮拜二 (“Tuesday,” considered the second day of the week), 二重唱 (“duet”)
- 兩黨 (“bipartisan”), 兩極 (“bipolar”), 海峽兩岸 (“cross-strait,” i.e. relating to China-Taiwan relations), 兩邊討好 (“to please both sides”), 兩棲動物 (“amphibian”)
For larger numbers containing the digit “2”:
- If “2” by itself (i.e., not as part of a larger number like “12” or “32”) is modifying 百 (“hundred”) or any larger power-of-ten word, use 兩. “Two thousand two hundred” is 兩千兩百. “Two hundred million” is 兩億.
- Otherwise, use 二. “Twenty-two” is 二十二. “222,222,222” is 兩億兩千兩百二十二萬兩千兩百二十二.
Now for the number “one.” It is always written with the same character, 一, but can be pronounced in three different ways: first tone (yī), second tone (yí), and fourth tone (yì).
As an example of the unhelpful explanations given by reference books, here’s what the Far East Pinyin Chinese-English Dictionary has to say:
Note: When yī precedes a 4th-tone sound, it is pronounced as yí. When the sound followed [sic] yī is a 1st-, 2nd-, or 3rd-tone sound, yī is pronounced as yì.
Since there are only four tones in the language, this makes it sound as if 一 is never pronounced as yī except when it is spoken in complete isolation, which is definitely not true.
What I suddenly realized one day (and what is, inexplicably, never explained in any dictionary or textbook I’ve seen) is that the rules for pronouncing 一 are basically the same as the rules for using 二 and 兩. Here they are:
- In situations where you would use 二, use first-tone yī.
- In situations where you would use 兩, follow the rules given in the dictionary. That is, use second-tone yí when a fourth-tone syllable follows, and fourth-tone yì otherwise. (This is the same rule as for 不.)
- As far as I can tell, all compound words and four-character idioms use first-tone yī. However, I can’t be entirely sure about that because dictionaries (and Google Translate) have an annoying habit of always transliterating 一 as yī, regardless of how it is actually pronounced.
So, those are the rules for “one” and “two,” so far as I’ve been able to figure them out. I welcome corrections from native speakers, questions from fellow students of the language, and textbook recommendations from anyone who knows a good one.