I put "He has several red birds" and was marked incorrect, I thought it was an OK translation!?!
"Several" would be "verscheidene" or "vele". "Enkele" kind of puts a limitation on the amount of birds this man has. When "enkele" is used there's not going to be massive amounts. Therefore "several" would be an overstatement and not a precise translation.
I'd say it is a correct translation, it just hasn't been put in the system as correct yet. Did you report it?
not a big difference and it would be considered correct. however usually 'een paar' would be 2 (a pair), so it's numerically more specific. but not always, and it's interchangeable.
no, 'sommige' is only as a part of a collection of things, people, etc. to clarify: a sentence with 'sommige' you could potentially complement with "andere." (others) (some cats are black, other cats are red / sommige katten zijn zwart, andere katten zijn rood).
It is "enkel" (singular) that means "single". An -e (not plural, but as a grammatical ending) is added to the word before a non-neuter noun:
Hij heeft een enkel bord = He has a single plate
Hij heeft een enkele vogel = He has a single bird
Hij heeft enkele vogels = He has a few birds
Hij heeft enkele bords = He has a few plates
Please correct me if I'm wrong.