"– Can you speak English? – No, I cannot."
Translation:– Osaatko puhua englantia? – En.
A complete negative answer to questions where -ko/kö is attached a verb is to form a compound verb(?) with the negative verb and the main verb.
- Osaatko puhua englantia? En osaa.
- Puhutko englantia? En puhu.
But these are often shortened to just the negative verb: en.
Well, according to the principles of Duolingo Finnish is here the original language, and the Finnish sentence uses singular you and I. That English uses "you" both for singular and plural and what it causes, has been taken up several times. IIUC the Finnish team has decided to accept both interpretations, but I don't know if this has been implemented thoughtout the course.
A general issue with these exercises is the lack of context. The context means surprisingly lot. However in this particular example asking with a plural you while getting a single person answer, I, would appear strange.
And I'm wrong anyway, because "osaatko" is second person singular, and cannot be interpreted as plural under normal circumstances. :)
They could always bypass the problem by using "thou" and "thee" as second person singular (but not plural) as it was in English 500 years ago, and then using "you" only as plural. Who knows - they might even start a trend!