Long and diphth.ongs.
The vowel sounds that are the most different in Middle English from their sound in Modern English are long vowels and diphth.ongs. The Great vowel shift from the mid 1300s to the 1700s changed the way many words were pronounced. Sheep was originally pronounced shep, but eventually it had a long e sound. Long vowel sounds happen when the vowel says it's name in a word.
For example, mate has a long A sound. Diphth.ongs happen when vowel sounds are combined. The first vowel sound slides into the second vowel sound. An example of a diphth.ong is boy. The pronunciation of the -oy sound slides from the o into the y sounds.