"A dog is eating bread."
Translation:כלבה אוכלת לחם.
@Botinok: The exercise at the top of this page is to translate from the English "A dog is eating bread" into Hebrew. The English sentence does not specify the dog's gender, so either gender should be OK. However, the standard translation to Hebrew would be masculine, "כלב אוכל לחם", when gender is unknown or unspecified, and they do accept that.
We could only wish that the course creators would explain why a preferred answer deviates from what the course teaches. Otherwise, they create the unnecessary confusion that you experienced.
DL gives you "typo" credit only for a 1-letter error that doesn't create another real and incorrect word.
Just as in English, we can't arbitrarily change spelling and expect the word to have the same meaning, even if it sounds the same. Examples: bread and bred; site and cite. In your error, the words don't mean or sound the same: "לֶחֶם" is lékhem; "לָכֶם" is lakhém.
Even native speakers (of both languages, for that matter) make errors in spelling, grammar, and pronunciation. As learners, we are guaranteed to make mistakes, but if the meaning is clear from context, then it might be okay in very casual correspondence and conversation.
Ultimately we must learn each noun's gender, but here are some general observations.
Many feminine nouns have the final letter ה preceded by an "ah" vowel; a few masculine nouns also have the final letter ה (but usually preceded by an "eh" vowel). For some animals the general name is masculine and that final letter ה preceded by an "ah" vowel distinguishes the feminine.
An adjective must match the gender and number of the noun that it modifies. For feminine adjectives, the final letter ה preceded by an "ah" vowel is pretty standard, and adding that ending is usually the way to form the feminine from the masculine.
The subject of Grammatical Gender is addressed in the Tips for the "Basics" skill, on the website. If you're on the website now, you can link to it at https://www.duolingo.com/skill/he/Basics/tips-and-notes