"I wear a sweater and a scarf."
Translation:Tôi mặc áo lạnh và khăn quàng.
"một" is implied. So unless you want to emphasize the number of sweaters and scarves you're currently wearing, no need to say "một."
Sigh… this kind of asymmetric translation is also frustrating in the English course for Chinese learners. Vietnamese, like Chinese, often omits the definite or indefinite articles (the, a,an) in context.
I used một but had to also use cái with both to be accepted. Why? I have no idea, just followed all my mistakes and corrections.