"They are three men."
Translation:वे तीन आदमी हैं।
They are generally interchangeable.
But here they are not.
They are three men denotes that you're not in very close proximity to the men. Thus वे तीन आदमी हैं।
ये तीन आदमी हैं। would be translated as These are three men and there would be an assumption that you're in close proximity to them.
only hard if you know only English and try to learn Hindi as second language... however Hindi has in grammatical structure lots in common with Latin. moreover the more languages you learned at highschool the more you feel at ease out of your mother tongue comfort zone. for now I know very well 3 germanic and 1 roman language, and know to pay attention to details when learning a new language, and not compare it anymore with my native language nor translate it in my head.
Thanks for your informative reply.
It would be great if it was possible to see other phrases in the Duolingo lessons broken down grammatically as sometimes it's not so simple as in your example above.
Here's a helpful five minute Hindi grammar lesson explaining Subject-Object-Verb (simple sentence structure) for others interested in this topic.
Though आदमी and आद्मी are both pronounced the same ("aadmii"), आदमी is the correct way of writing the word. This is because the word is derived from Arabic/Persian 'aadami' which in turn comes from the Arabic pronunciation of 'Adam'.
The vowel after द is left out in the Hindi pronunciation, not because the word has a consonant combination of द and म (as in आद्मी) but because of the phenomenon of Schwa deletion.
Incidentally, Adam (as in the Biblical/Koranic character) is written as आदम in Hindi and pronounced 'Aadam' because schwa deletion rules don't call for the deletion of the vowel after द in this word.