Grammar: The pronunciation of "ae" as "e"

One of the most important sound changes in Latin was the gradual disappearance of the diphthong "ae", which came to be pronunced simply as "e". In some words, the spelling did not change (but the pronunciation probably did!) -- and in other cases, the spelling of the word changed to reflect the pronunciation.

This change affected the stem of the word: so, for example, the word haereo was often spelled hereo (which is, ultimately, why we say "adhesive" in English, instead of "adhaesive"). This change also affected the morphological endings of words, especially in the feminine declension: the plural of femina was no longer feminae, but femine.

This change in the pronunciation of the diphthong "ae" was one of the most destabilizing factors in the declension system of Latin nouns. If you know Spanish or Italian or French, you can see that the Latin verb system survived more or less intact in these languages, but the declension of nouns basically comes to a complete halt. This was because people could no longer hear the difference between the different word endings. The confusion between "ae" and "e" was one of the biggest factors in this breakdown in the noun declensions.