The future tense of third and fourth conjugation verbs is marked by -ē-, as in trahes and audies. The regular personal endings are added after this vowel. But in the first person singular the vowel is changed; it is traham and audiam instead of *trahem and *audiem.
Why is this? Words ending in -em are by no means forbidden in Latin, so this strikes me as very odd. What is the origin of this different vowel? Are similar changes observed elsewhere in Latin? Any help in making sense of this vowel change would be great.