When to use e vs ed?
To reinforce what "giulap" said, typically 'ed' is not used when speaking, but is used when writing.
Whatever the correct usage, it's very frustrating when a pop up box tells you that using e or ed is largely a matter of preference, then you immediately get marked incorrect for using ed instead of e.
"Ed" is used when the following word begins with a vowel. It's optional when you want to separate vowel sounds. Check out the link below for a more in-depth explanation.
I'd like to add that it seems Duolingo doesn't believe it is optional. They are also strict on the usage.
Mmm I disagree, I never use "ed" when I speak. Example "e il cane?" sounds like "el cane?". I usually use "ed" when I am writing something formal, or every now and then in informal writing.
Actually, "ed" is used when the following word begins with "e". http://dizionari.corriere.it/dizionario-si-dice/A/a-ad.shtml