"I want to eat a sandwich."
Translation:Yo quiero comer un sándwich.
You have two verbs in a row here: want and eat. The first one gets conjugated (I want = yo quiero), and the second one doesn't (to eat = comer). That's pretty much the rule when you have two verbs together. English does the same thing.
You want (tú quieres) to work (trabajar) = Tú quieres trabajar.
He needs (él necesita) to study (estudiar) = Él necesita estudiar.
If you translate Duo's sentence as [Yo quiero como], you would be saying something like [I want I eat].
Why has sandwich replaced emperadado? The latter is such a great word, one of my favourites!
If you want to help expand the database, keep using emparedado. If it's marked wrong, use the Report button to click "my answer should be accepted." I believe it will eventually be added back in.
Depending on context (which a singular phrase like this doesn't have), I believe that the variations "Me gustaría (comer) un sándwich/emparedado/bocadillo" are also correct, never mind how common or regional those variants would be.