A new option "Correct Tone for 不/一" has been added in our pinyin converter tool, which provides correct pinyin tones for Chinese character 不 and 一 based on the context.

The rules we used are as below:
 

For 不 (bù):

1. 不 is pronounced with second tone when followed by a fourth tone syllable. For example, 不对 bú duì
2. When 不 is used between words in an A-not-A question, it may become neutral in tone (e.g. 是不是 shì bu shì).
3. In other cases, 不 is pronounced with fourth tone bù. 

For 一 (yī):

1. 一 is pronounced with second tone when followed by a fourth tone syllable. For example, 一面 yí miàn
2. If it is before a first, second or third tone syllable, 一 is pronounced with fourth tone. For example, 一无所有 yì wú suǒ yǒu.
3. When 一 is the final character, or when it comes at the end of a multi-syllable word (regardless of the first tone of the next word), 一 is pronounced with first tone. For example，统一思想 tǒng yī sī xiǎng
4. It also has first tone when used as an ordinal number (or part of one, e.g. 第一名 dì yī míng), and when it is immediately followed by any digit (including another 一; hence both syllables of the word 一 一 yī yī  and its compounds have first tone).
5. When 一 is used between two reduplicated words, it may become neutral in tone (e.g. 看一看).