I had my answer rejected for writing "Birds can fly but they cannot swim"! From Oxford dictionary: "Both cannot and can not are acceptable spellings, but the first is much more usual." It seems like there are a lot of these types of errors in the Hindi Duolingo, but not in other languages like Spanish and French -- would be great to get them corrected!
"Birds can fly but not swim" was marked wrong. (I have taught English, I know it is correct) Instead of trying to teach literal translation in bad English (no one says "can not" - even in India as far as I know - it is cannot), it would be more helpful if they also got the English usage right.