"We eat an apple."
अनुवाद:हम एक सेब खाते हैं।
There is a quite a lot of ambiguity with regard to the use of articles in Hindi. Since there is no definite article "the" in Hindi, there's no question about that. But with regard to the indefinite articles "a" and "an", their use is quite optional in Hindi. For example, to say "I am a boy", saying "मैं लड़का हूँ।" is equally correct to saying "मैं एक लड़का हूँ।" Of course, there are some sentences in which you have to use the article "एक", but otherwise, it's okay, even if you don't use it.
Thank you for your comment! I like it but we are learning English language so why Duolingo team is given important to Hindi grammar, when we are being writing Hindi correctly only one mistake like full stop, without point on its word's head, it is normally small mistake in our Hindi writing but the Duolingo marks wrong sentence...
The Hindi team is not focusing on Hindi grammar, but there are certain nuances in each language that we need to take into consideration. Also, Duolingo doesn't penalise you for not using punctuations, like full stops, commas etc. Its left up to you, whether you want to use them or not. Regarding the अनुस्वार ( ं ), if you don't put it above "है" it would be "is" and when you put it, it means "are". So if you say "हम आदमी है" it is like saying "We is men". Therefore, it is not accepted. I know this is confusing even for natives, but with a little practice, I'm surely you'll eventually understand which to use when.