"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."
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.
It is not consistant. Sometimes một sometimes not.... what is it? Makes me wanna stop these lessons...
"áo ấm" is also acceptable. I am native speaker and I use "áo ấm" more than "áo lạnh"