The numeral 一yī may be pronounced three different ways:
1. **一**
   When 一 is used alone, or stands at the end of a sentence, or is used as part of an ordinal number.
   十一
   shíyī
   Eleven
2. **一 + (4th tone)**
   When 一 is followed by a fourth tone, it takes the second tone.
   一件
   yíjiàn
   One (mw. clothing item)
3. **一 + (1st/2nd/3rd tone)**
   When 一 is followed by other tones, it takes the fourth tone.
   一本
   yìběn
   One (mw. book)
4. **一 + (neutral tone)**
   If the neutral toned syllable was originally 4th tone, then (2) applies - e.g. yígè -> yíge
   Else, (3) applies