Answer:
We use it when the last letter of the word that connects to another ends with i or y, so it doesn't sound bad.
Explanation:
Example: Hoy e inicié mis clases.
The same goes for the o, or u. The reason why "u" is used instead of "o" is to avoid a cacophony, which also occurs when saying "o ocho"
It is only to avoid repeating the vowels.
