How to Properly Pronounce “一” In Different Contexts

We talked before here about the “Tone Sandhi”, or tone change rules, for the Chinese 3rd Tone. There is another one of these rules for the character that most people learn before any other, “一” yī. We learn this first as the character representation of the number one.
Here’s the rule:

  1. 一 (yi1) tone changes
    1. Only when it’s a number is it pronounced as a 1st tone
    2. Before a 1st, 2nd, or 3rd tone, “一” changes to a 4th tone
    3. Before a 4th tone, “一” changes to a 2nd tone

This rule exists to make a sentence naturally flow better, and I bet you will start to bring greater attentiveness to it once you realize its in loads of common words.

As a Number
This is the tone set in the dictionary APP Pleco, and it is correct when referring to a number, but the majority of the time that “一” is used in day to day life is in non-numerical words, so let’s look at some of those.
Before a 1st, 2nd, or 3rd tone
By changing the tone of “一” to 4th tone, it makes these words flow more easily in sentences.

Before 4th Tone
Second tone followed by a first tone is very natural to say, even foreigners tend to not struggle with it too much, so its understandable why this rule developed to make sentence flow better. Also, some of you are probably wondering why 一个 is pronounced yíge5 when “ge5” is 5th tone, not 4th tone. The reason is because “个” is a 4th tone when used by itself, but at the language naturally evolved, using “个” in the context of measure words turned into a 5th tone, but the tone change for “一” didn’t change.
Screenshots from Pleco , an awesome Chinese-English Dictionary APP

NOTE: Pleco (incredible as it is as a study tool) does NOT make note of any of these important tone changes. Depending on which version you have, it might just put a little dot under the pinyin for “一” (yī), but you may not even be that lucky! It always writes as first tone, and this is wrong. Check out the picture below to see what we mean:
Keep looking out for your “一” tone change rules, and you’ll sound like a native in no time :D.