My dad told me that it should be "một chiếc xe đạp". He's from the South if that helps. Wiktionary says "As a classifier, chiếc is usually considered as a formal, or even literary, counterpart to cái. It is mostly used colloquially when indicating some types of vehicles such as bicycles, motorcycles, cars, etc."
<Chiếc> can be more formal and more suitable for "xe đạp". However, Duolingo team has decided to use <cái> as the only classifier for almost all household items (at the very beginning levels at least).
Generic classifier, applies to nearly everything. I think one of the conditions is "tangible lifeless entity".
Is cái a noun classifier? From what I remember on a different site, the appropriate pronoun was chiec. Can anyone clarify?
Both work here. I like chiếc more. I cannot explain why but chiếc often uses for things you like, things you would treat it with a bit of care, and there are only a few things you can use chiếc with.
American English, I guess? Did you try typing "bicycle"? How was the result?
Bicycle works just fine. "Bike" is listed as alternate answer. Probably user's own native language setting?
When I typed in the answer as bicycle they said it was wrong. But bicycle is the same thing as a bike. So I shouldn't be marked wrong.