"I bought a computer."
Translation:Αγόρασα έναν ηλεκτρονικό υπολογιστή.
is it έναν because the next word begins with a vowel and is in the accusative?
According to the spelling rules currently taught in schools, the form is έναν because the next word is (a) masculine and (b) in the accusative case.
According to the spelling rules I learned, the form is έναν because the next word is (a) masculine and (b) in the accusative case and (c) starts with a vowel.
I learned to write, for example, Βλέπω ένα σκύλο for "I see a dog" but the current rules say it should be Βλέπω έναν σκύλο -- the final nu of έναν is always retained, regardless of what sound the next word starts with.
You may find some of this older usage in the course, since all of us grew up with the old rules; but we're trying to move towards the current standard school spelling.