Change in tones for "一" and "不"

Most Chinese characters has one unique pinyin for itself. However, some characters has more than one pinyin. The character 一 and 不 change their tones depends on what follows behind them. 

The basic tone of numeral "一” is 1st tone (yī)．When read alone, or in counting or in numbers, its basic tone is used. 

The tone of "一“ may vary with the tone of the syllable that comes after it: if followed by the 1st, 2nd or 3rd tone, "一” is pronounced as the 4th tone; if it is followed by a "4th" tone, it is pronounced as the 2nd tone. 


Example: 
yī ,   èr,  sān
一， 二， 三 (one, two, three) 
yì   tiān  
 一  天     (one day)
yì   nián
一    年    (one year)
yì   běn
一   本     ( one [measure word for books])
yí   gè
一  个     ( one [measure word ge])


Similar rule applies to the character "不"
bù zhī dào
不  知   道     (don't know) 
bù míng bai
不  明     白      (don't understand)
bù hǎo
不  好   (not good)
bú shì
不  是   (no)