Yi Bu Yi Bu Lai (Tone Change Rules for 一 不) & phrase 一步一步来吧

Here is an explanation for the tone changes for 一 yī and 不 bù, which both follow a similar pattern.

一 yī is first tone whenever it is used as a number but changes based on the tone immediately following it, in the following pattern.

一 becomes 2nd tone when followed by a 4th tone.

一 becomes 4th tone when followed by 1st, 2nd or 3rd tone.

For example, if you say 第一次 Dì yīcì (the first time) it remains a first tone because it is used as a number and you're emphasizing the first time. However if you say 一次 yí cì it changes to 2nd tone because you're then talking about 'a time'.

The tone change for 不 bù is more common, and less confusing, because you don't have to worry about the 'as a number' issue. 不 bù is normally a 4th tone but changes the same way before a 4th tone.

不 becomes 2nd tone when followed by 4th tone.

不 stays 4th tone when followed by 1st, 2nd or 3rd tone.

For example, 不知道 bù zhī dào - bù is 4th because zhī is 1st. However it changes to 2nd tone if you say 不是 bú shì.

Since tone changes can be a problem, here's a useful phrase to learn at the same time: 一步一步来吧 - yí bù yí bù lái ba. The phrase means more or less 'step by step' and implies that progress takes time. When followed with ba it can be used to encourage people. For example say that someone is struggling with Chinese grammar & frustrated, you could say "一步一步来吧" saying that you're making progress but it takes time. Or if someone asks how your Mandarin is coming, you could say "一步一步来", meaning "it's coming along, slowly, step by step".

Please correct me if I have made any mistakes in my post. This is based on my teacher's explanation to me.

(Beyonce photo from fashion-police.ca)