The Great Vowel Shif

english prefers the i before e spelling for the e sound. A few centuries after the great vall shift, there was another more limited vowel shift that took place in and around london. That later london shift helped to define the local cockney accent. This particular aspect of cockney speech was famously featured in the film, my fair lady. It explains why cockney speakers sometimes pronounce day as something like di,. and train as trin and n as spine.