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.
一 is pronounced with second tone when followed by a fourth tone syllable. For example, 一面 yímiàn
If it is before a first, second or third tone syllable, 一 is pronounced with fourth tone. For example, 一无所有 yìwúsuǒyǒu.
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 tǒngyīsīxiǎng
It also has first tone when used as an ordinal number (or part of one, e.g. 第一名dìyīmíng), and when it is immediately followed by any digit (including another 一; hence both syllables of the word 一 一yīyīand its compounds have first tone).
When 一 is used between two reduplicated words, it may become neutral in tone (e.g. 看一看).
